Pin PR resource to v0.21 to avoid Github abuse rate limit

We started hitting this on Thursday, and there's been ongoing report
from the community about this as well. While upstream is figuring out a
long term solution [1], we've been advised [2] to pin to the previous
release (v0.21.0) to avoid being blocked for hours at once.

[1]: https://github.com/telia-oss/github-pr-resource/pull/238
[2]: https://github.com/telia-oss/github-pr-resource/pull/238#issuecomment-714830491

concourse/pipelines/pr_pipeline.yml

@@ -7,6 +7,7 @@ resource_types:
   type: registry-image
   source:
     repository: teliaoss/github-pr-resource
+    tag: v0.21.0
 - name: gcs
   type: registry-image
   source: