MinimumRequirements: update minimum requirements for riscv64

For golang/go#61476

Change-Id: Ibf656561613eb016f82c31248bf09300d012218a
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/696655
Reviewed-by: Sean Liao <sean@liao.dev>

MinimumRequirements.md

diff --git a/MinimumRequirements.md b/MinimumRequirements.md
index 900390b..61863c5 100644
--- a/MinimumRequirements.md
+++ b/MinimumRequirements.md
@@ -158,7 +158,8 @@
 
 ### riscv64
 
-rv64g (rv64imafd)
+rv64g (rv64imafd).
+Starting from Go 1.23, RVA20U64 mandatory extensions is required.
 
 ### loong64