Update doc link

Author: benjyw

.github/workflows/pants.yaml

@@ -1,7 +1,7 @@
 # Copyright 2022 Pants project contributors.
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-# See https://www.pantsbuild.org/2.21/docs/using-pants/using-pants-in-ci for tips on how to set up your CI with Pants.
+# See https://www.pantsbuild.org/stable/docs/using-pants/using-pants-in-ci for tips on how to set up your CI with Pants.
 
 name: Pants
 
@@ -76,7 +76,7 @@ jobs:
           # Note that named_caches and lmdb_store falls back to partial restore keys which
           # may give a useful partial result that will save time over completely clean state,
           # but will cause the cache entry to grow without bound over time.
-          # See https://www.pantsbuild.org/2.21/docs/using-pants/using-pants-in-ci for tips on how to periodically clean it up.
+          # See https://www.pantsbuild.org/stable/docs/using-pants/using-pants-in-ci for tips on how to periodically clean it up.
           # Alternatively you change gha-cache-key to ignore old caches.
 
       - name: Bootstrap Pants