ci: bump cf-deployment-concourse-tasks from v15 to v20

bbl 9.0.41 (bundled in v20) ships bosh-dns 1.39.18 with
systemd-resolved support required for ubuntu-noble stemcells.
The v15 image used bbl 8.x with bosh-dns 1.35.1 which lacks
noble support, causing bosh-dns post-start failures on deploy-cf.

Author: bonzofenix

ci/infrastructure/pipeline.yml

@@ -103,7 +103,7 @@ resources:
   source:
     branch: main
     uri: https://github.com/cloudfoundry/cf-deployment-concourse-tasks.git
-    tag_filter: v15.*
+    tag_filter: v20.*
 
 - name: gcp-noble-stemcell
   type: bosh-io-stemcell