Internalize PostgREST mirror workflow

[jgoux]

Summary

• replace the supabase/cli reusable mirror workflow in mirror-postgrest.yml
• mirror docker.io/postgrest/postgrest:v<version> directly from this repo
• push the mirrored tag to public.ecr.aws/supabase/postgrest:v<version> and ghcr.io/supabase/postgrest:v<version>

Notes

The Postgres image release workflows already call this repo’s local mirror.yml. This change removes the remaining cross-repo mirror dependency for the PostgREST image.

Validation

• parsed .github/workflows/mirror-postgrest.yml as YAML
• ran git diff --check

[jgoux]

Actually I'm not sure it's the best move, it would be better to have this process directly internalized in PostgREST just after they deploy their docker image instead of waiting for someone to update PostgREST version in here to trigger this workflow.
On the CLI side, dependabot is updating the local stack image versions and we get failures because dependabot is faster than the human updating the PostgREST version in this repo so the image of PostgREST isn't mirrored yet when our CI runs against the dependabot's PR.