bump immich to beta-rc3 tag

[jtrotsky]

Bump immich-server beta to keep in line with upstream which is now at rc3 https://github.com/immich-app/immich/releases/tag/v3.0.0-rc.3

Tests

• dbuild build --no-cache succeeds
• verify dbuild info shows rc3
• dbuild test succeeds

=== Image: ghcr.io/jtrotsky/immich-server ===
[info] Title: Immich Server
[info] Type: app (Photos & Media)
[info] Architectures: amd64
[info] Variants: 2
[info]   latest (amd64) -> Containerfile
[info]   beta (amd64) -> Containerfile
[info]     UPSTREAM_URL=https://api.github.com/repos/immich-app/immich/releases/tags/v3.0.0-rc.3
[info] Test: mode=shell port=None
[info] $ doas podman images --format json --filter reference=ghcr.io/jtrotsky/immich-server
[info] Build cache:
[info]   :build-beta    3.4 GB  1m ago  v3.0.0-rc.3
[info]   :build-latest  3.4 GB  2m ago  v2.7.5

=== Build summary ===
[ok]   ghcr.io/jtrotsky/immich-server:build-latest
[ok]   ghcr.io/jtrotsky/immich-server:build-beta

=== Test summary ===
  :latest   [ok]
  :beta     [ok]
[ok] 2/2 passed