internal/task: use Go1.N milestone for go1.N.0 major release

We're still using Go1.N milestones to track major Go release development
and need to update FetchMilestones to handle that as of go1.21.0, the .0
version component is explicitly included in the major version string.

For golang/go#57631.

Change-Id: I1dcc366796af2e3831ff866e957bcbb0967bdfe5
Reviewed-on: https://go-review.googlesource.com/c/build/+/515036
Run-TryBot: Dmitri Shuralyov <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: dmitshur

internal/task/milestones.go

@@ -55,8 +55,8 @@ func (m *MilestoneTasks) FetchMilestones(ctx *wf.TaskContext, currentVersion str
 	}
 	majorVersion := fmt.Sprintf("go1.%d", x)
 
-	// RCs and betas use the major version's milestone.
-	if kind == KindRC || kind == KindBeta {
+	// Betas, RCs, and major releases use the major version's milestone.
+	if kind == KindBeta || kind == KindRC || kind == KindMajor {
 		currentVersion = majorVersion
 	}