dashboard: enable linux-arm64-longtest builder on 1.20+ release branches

From the dashboard, it seems the linux-arm64-longtest builder has
been working reasonably well for a while. At this point we don't
have known issues for it. So remove known issues.

Enable it for trybot on 1.20+ release branches, per golang/go#37827.
Only for 1.20+ because it is failing on older versions of Go.

Fixes golang/go#49649.
Updates golang/go#53851, golang/go#37827.

Change-Id: I682aa4028b791ecd80360fbe05ef615dad19405d
Reviewed-on: https://go-review.googlesource.com/c/build/+/456043
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
diff --git a/dashboard/builders.go b/dashboard/builders.go
index 04a1157..6782e0a 100644
--- a/dashboard/builders.go
+++ b/dashboard/builders.go
@@ -2663,13 +2663,12 @@
 		Name:     "linux-arm64-longtest",
 		HostType: "host-linux-arm64-bullseye-high-disk",
 		Notes:    "Debian Bullseye with go test -short=false",
-		// TODO: make it trybot on release branch once we know it works.
-		//tryBot: func(repo, branch, goBranch string) bool {
-		//	onReleaseBranch := strings.HasPrefix(branch, "release-branch.")
-		//	return repo == "go" && onReleaseBranch // See issue 37827.
-		//},
+		tryBot: func(repo, branch, goBranch string) bool {
+			onReleaseBranch := strings.HasPrefix(branch, "release-branch.")
+			return repo == "go" && onReleaseBranch // See issue 37827.
+		},
 		buildsRepo: func(repo, branch, goBranch string) bool {
-			b := buildRepoByDefault(repo)
+			b := atLeastGo1(goBranch, 20) && buildRepoByDefault(repo)
 			if repo != "go" && !(branch == "master" && goBranch == "master") {
 				// For golang.org/x repos, don't test non-latest versions.
 				b = false
@@ -2681,7 +2680,6 @@
 			"GO_TEST_TIMEOUT_SCALE=5", // give them lots of time
 		},
 		numTryTestHelpers: 4, // Target time is < 15 min for go.dev/issue/42661.
-		KnownIssues:       []int{53851, 49649},
 	})
 	addBuilder(BuildConfig{
 		Name:              "linux-arm-aws",
diff --git a/dashboard/builders_test.go b/dashboard/builders_test.go
index d662265..baff418 100644
--- a/dashboard/builders_test.go
+++ b/dashboard/builders_test.go
@@ -127,6 +127,46 @@
 		},
 		{
 			repo:   "go",
+			branch: "release-branch.go1.20",
+			want: []string{
+				"freebsd-amd64-12_3",
+				"js-wasm",
+				"linux-386",
+				"linux-amd64",
+				"linux-amd64-boringcrypto",
+				"linux-amd64-race",
+				"linux-arm-aws",
+				"linux-arm64",
+				"openbsd-amd64-70",
+				"windows-386-2008",
+				"windows-386-2012",
+				"windows-amd64-2016",
+
+				"misc-compile-darwin",
+				"misc-compile-freebsd",
+				"misc-compile-windows-arm",
+				"misc-compile-mips",
+				"misc-compile-mipsle",
+				"misc-compile-netbsd",
+				"misc-compile-netbsd-arm",
+				"misc-compile-openbsd",
+				"misc-compile-openbsd-arm",
+				"misc-compile-plan9",
+				"misc-compile-ppc",
+				"misc-compile-solaris",
+				"misc-compile-other-1",
+				"misc-compile-other-2",
+				"misc-compile-go1.20",
+
+				// Include longtest builders on Go repo release branches. See issue 37827.
+				"linux-386-longtest",
+				"linux-amd64-longtest",
+				"linux-arm64-longtest",
+				"windows-amd64-longtest",
+			},
+		},
+		{
+			repo:   "go",
 			branch: "release-branch.go1.18",
 			want: []string{
 				"freebsd-amd64-12_3",
@@ -159,45 +199,6 @@
 				// Include longtest builders on Go repo release branches. See issue 37827.
 				"linux-386-longtest",
 				"linux-amd64-longtest",
-				//"linux-arm64-longtest",
-				"windows-amd64-longtest-oldcc",
-			},
-		},
-		{
-			repo:   "go",
-			branch: "release-branch.go1.17",
-			want: []string{
-				"freebsd-amd64-12_3",
-				"js-wasm",
-				"linux-386",
-				"linux-amd64",
-				"linux-amd64-race",
-				"linux-arm-aws",
-				"linux-arm64",
-				"openbsd-amd64-70",
-				"windows-386-2008-oldcc",
-				"windows-386-2012-oldcc",
-				"windows-amd64-2016-oldcc",
-
-				"misc-compile-darwin",
-				"misc-compile-freebsd",
-				"misc-compile-windows-arm",
-				"misc-compile-mips",
-				"misc-compile-mipsle",
-				"misc-compile-netbsd",
-				"misc-compile-netbsd-arm",
-				"misc-compile-openbsd",
-				"misc-compile-openbsd-arm",
-				"misc-compile-plan9",
-				"misc-compile-ppc",
-				"misc-compile-solaris",
-				"misc-compile-other-1",
-				"misc-compile-other-2",
-
-				// Include longtest builders on Go repo release branches. See issue 37827.
-				"linux-386-longtest",
-				"linux-amd64-longtest",
-				//"linux-arm64-longtest",
 				"windows-amd64-longtest-oldcc",
 			},
 		},