Promote postgres 17.9-percona image tag

[souravbiswassanto]

Adds library/postgres/promote_tags.txt so the promote workflow copies the
Percona Server for PostgreSQL 17.9 + pg_tde image out of the daily registry.

17.9-percona_20260807

cmd/promote strips everything after the last _ to derive the destination tag,
so this one line produces:

crane cp ghcr.io/appscode-images/daily/postgres:17.9-percona_20260807 \
         ghcr.io/appscode-images/postgres:17.9-percona

Same shape as the -ext promotion in 4806a88.

Verified before opening

• the source image exists and is multi-arch — crane manifest on
  ghcr.io/appscode-images/daily/postgres:17.9-percona_20260807 reports
  linux/amd64 + linux/arm64
• it came from a green build: Build (17.9-percona) succeeded in
  https://github.com/appscode-images/builder/actions/runs/31191190472 (the run on
  master right after PostgreSQL Percona pg_tde (TDE) image support #109 merged)
• the timestamp matches what the build actually pushed, taken from that job's log
  rather than assumed
• parsing checked against the real file through lib.ListPromoteTags, which
  resolves the exact crane cp pair shown above

Note on the filename

The file is promote_tags.txt, which is what lib.FilePromotedTags reads — not
promote.txt. Worth being precise here since two earlier commits (abd220e,
936fa52) were fixes for this filename being slightly off.

Once the promote workflow runs, cmd/promote deletes this file and commits the
removal, so it is expected to be short-lived on master.

Depends on: #109 (merged)