feat(rel): add backport GHA to Jetbrains (#2650)

Having tested backport functionality in the [Cody
repo](sourcegraph/cody#6119) I'm now adding it
here too.

This PR adds a backport github action based our backport tooling in
sourcegraph/backport
It uses the almost the same label as the Cody one, but with a slightly
different base branch (backport jetbrains-v<version-number>). Notably,
the `jetbrains-v<version-number>` branch doesn't appear to be the
standard here, but I think we should adopt the same convention that is
used in Cody.

## Test plan
CI
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---------

Co-authored-by: Kalan <[email protected]>

.github/workflows/backport.yml

@@ -0,0 +1,27 @@
+name: Backport
+on:
+  pull_request_target:
+    types:
+      - closed
+      - labeled
+
+jobs:
+  backport:
+    name: Backport
+    runs-on: ubuntu-latest
+    # Only react to merged PRs for security reasons.
+    # See https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target.
+    if: >
+      github.event.pull_request.merged
+      && (
+        github.event.action == 'closed'
+        || (
+          github.event.action == 'labeled'
+          && contains(github.event.label.name, 'backport')
+        )
+      )
+    steps:
+      - uses: sourcegraph/backport@v2
+        with:
+          github_token: ${{ secrets.BACKPORT_GITHUB_TOKEN }}
+          label_pattern: '^backport (?<base>jb-v\d+\.\d+\.x)$'