.github/workflows: only release on 2xxx-xx-xx tags

I will add an 'abseil.old' tag preserving the Abseil branch history.
This avoids having that tag trigger a release action.

Change-Id: I3d44ff9d91bf205bd5425adcb4b1da044a531661
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63831
Reviewed-by: Alan Donovan <adonovan@google.com>

.github/workflows/release.yml

diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 978710e..56c9d23 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -1,7 +1,7 @@
 name: Release
 on:
   push:
-    tags: ['**']
+    tags: ['2[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9]'] # yyyy-mm-dd
 permissions:
   contents: read
 jobs: