…ate main target bug (#1231)

Closes REL-1067

## Problem

The release pipeline has been systematically failing in the
`promoteToPublic.finalize` workflow with this error:
```
Warning: 1 uncommitted change
must be on a branch named differently than 'promote/release-v6.4.3889-update-main'
```

## Root Cause

The final `github:pr` step in the `promoteToPublic.finalize` workflow
was incorrectly trying to create a PR from the current branch to itself:

- **Current branch**: `promote/release-{{version}}-update-main`
- **Target branch**: `promote/release-{{version}}-update-main` ❌ (same
branch!)
- **Should target**: `main` ✅

This happened because the variable `internal_branch` was set to the
current branch name instead of the intended target branch.

### Test plan
This will have to be tested on a release
 <br> Backport d7ddf22 from #1227

Co-authored-by: Warren Gifford <warrenbruceg@gmail.com>

…amp registry (#1261)

## Problem
The release creation process was failing with 404 errors when trying to
fetch images like cadvisor:6.6.2517 from the legacy internal registry.

## Root Cause
Recent changes in the main sourcegraph repo migrated from legacy
registries to new 'revamp' registries, but the deploy repos were
still hardcoded to use the old registry.

## Solution
- Replace `us-central1-docker.pkg.dev/sourcegraph-ci/rfc795-internal`
with `us-docker.pkg.dev/sourcegraph-images/internal`
- Updated both release.yaml and batch-change/release.yaml
- Tested and confirmed images exist in the new registry

## Test Plan
- Validated that cadvisor:6.6.2517 exists in new registry but not in old
registry
- Successfully tested sg ops update-images command with new registry
- Confirmed this fixes the original 404 errors during release creation
<br> Backport 5b81ee7 from #1260

Co-authored-by: Warren Gifford <warrenbruceg@gmail.com>
Co-authored-by: Amp <amp@ampcode.com>