internal/http3: include minor HTTP version in default User-Agent header

HTTP/1 and HTTP/2 transports include the minor version in their default
User-Agent header. Do so for HTTP/3 too for consistency.

For golang/go#70914

Change-Id: I750bf9a21bef28594262c33349940d856a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/817121
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>

internal/http3/roundtrip.go

diff --git a/internal/http3/roundtrip.go b/internal/http3/roundtrip.go
index c5aab72..46afe67 100644
--- a/internal/http3/roundtrip.go
+++ b/internal/http3/roundtrip.go
@@ -143,7 +143,7 @@
 			},
 			AddGzipHeader:         addedGzip,
 			PeerMaxHeaderListSize: 0,
-			DefaultUserAgent:      "Go-http-client/3",
+			DefaultUserAgent:      "Go-http-client/3.0",
 		}, func(name, value string) {
 			// Issue #71374: Consider supporting never-indexed fields.
 			yield(mayIndex, name, value)

internal/http3/roundtrip_test.go

diff --git a/internal/http3/roundtrip_test.go b/internal/http3/roundtrip_test.go
index 2ca08ae..814233b 100644
--- a/internal/http3/roundtrip_test.go
+++ b/internal/http3/roundtrip_test.go
@@ -902,7 +902,7 @@
 				":method":    []string{"GET"},
 				":path":      []string{"/"},
 				":scheme":    []string{"https"},
-				"User-Agent": []string{"Go-http-client/3"},
+				"User-Agent": []string{"Go-http-client/3.0"},
 			}
 			tt.setup(tc, req, wantHeaders)
 			tc.greet()