Bump snok/container-retention-policy from 1.4.1 to 2.1.2

Bumps [snok/container-retention-policy](https://github.com/snok/container-retention-policy) from 1.4.1 to 2.1.2.
- [Release notes](https://github.com/snok/container-retention-policy/releases)
- [Commits](snok/container-retention-policy@v1.4.1...v2.1.2)

---
updated-dependencies:
- dependency-name: snok/container-retention-policy
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/prune-untagged-images.yml

@@ -23,7 +23,7 @@ jobs:
           - met-upperair
     steps:
       - name: Delete image
-        uses: snok/container-retention-policy@v1.4.1
+        uses: snok/container-retention-policy@v2.1.2
         with:
           image-names: metexpress/development/${{ matrix.app }}
           cut-off: 1 day ago MST