CI - Release branch check auto-changes base branch (#298)

## Description of Changes
Instead of our CI failing and telling the user that a PR should be based
on `master`, we automatically change the base branch to `master`.

## API

No code changes.

## Requires SpacetimeDB PRs
None

## Testsuite
SpacetimeDB branch name: master

## Testing
- [x] See below, where the new CI changed the base branch of this PR!
- [x] On this test PR, the base branch was not changed:
#299

---------

Co-authored-by: Zeke Foppa <[email protected]>

Author: bfops

.github/workflows/check-pr-base.yml

@@ -3,20 +3,29 @@ name: Git tree checks
 on:
   pull_request:
     types: [opened, edited, reopened, synchronize]
-  merge_group:
-permissions: read-all
+
+permissions:
+  pull-requests: write
+  contents: read
 
 jobs:
   check_base_ref:
     name: Release branch restriction
     runs-on: ubuntu-latest
     steps:
-      - id: not_based_on_master
+      - name: Change base to master if needed
         if: |
           github.event_name == 'pull_request' &&
           github.event.pull_request.base.ref == 'release/latest' &&
           ! startsWith(github.event.pull_request.head.ref, 'release/')
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
-          echo 'Only `release/*` branches are allowed to merge into the release branch `release/latest`.'
-          echo 'Maybe you want to change your PR base to `master`?'
-          exit 1
+          echo 'Updating PR '"#${{ github.event.pull_request.number }}"' to use base `master`'
+          echo 'If you do want to release your changes directly by merging into `release/latest`, your branch must start with `release/`.'
+
+          curl -X PATCH \
+            -H "Authorization: Bearer $GITHUB_TOKEN" \
+            -H "Accept: application/vnd.github+json" \
+            https://api.github.com/repos/${{ github.repository }}/pulls/${{ github.event.pull_request.number }} \
+            -d '{"base": "master"}'