x/tools/gopls/internal/regtest/workspace: TestQuickFix_AddGoWork fails with released go 1.21+

[hyangah]

Go version go1.21rc2

The behavior of the go command is changing, and the test needs fixing to work with go1.21+.

#### End Gopls Test Logs for "TestQuickFix_AddGoWork/use_all/experimental"
--- FAIL: TestQuickFix_AddGoWork (22.95s)
    --- FAIL: TestQuickFix_AddGoWork/use_b (8.02s)
        --- FAIL: TestQuickFix_AddGoWork/use_b/default (3.05s)
            quickfix_test.go:208: unexpected go.work content:
                --- want
                +++ got
                @@ -1,4 +1,4 @@
                -go 1.21
                +go 1.21rc2
                 
                 use ./b
...
FAIL
FAIL    golang.org/x/tools/gopls/internal/regtest/workspace     233.461s

[gopherbot]

Change https://go.dev/cl/508801 mentions this issue: gopls/internal/regtest/workspace: fix TestQuickFix_AddGoWork for go1.21