ci: improve backport PR title and body (#420)

The previous templates were not compatible with our usage:

- The title was prefixed with `[Backport release-1.x]` which broke
`release-please`
- The body was empty besides the comment about backporting. By default
we use the PR body as the commit message, so we should include the
relevant details here.

(cherry picked from commit 2e44ada)

.github/workflows/backport.yml

@@ -21,6 +21,19 @@ jobs:
         )
       )
     steps:
-      - uses: tibdex/backport@v2
+      - uses: jooola/backport@main
+        # Using a fork of tibdex/backport@v2 to get https://github.com/tibdex/backport/pull/109
         with:
-          github_token: ${{ secrets.GITHUB_TOKEN }}
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+
+          # These templates are compatible with release-please and our default commit body (body of PR).
+          title_template: "<%= title %> [Backport <%= base %>]"
+          body_template: |
+            <%= body %>
+            
+            ---
+            Backport <%= mergeCommitSha %> from #<%= number %>.
+            
+            BEGIN_COMMIT_OVERRIDE
+            <%= title %>
+            END_COMMIT_OVERRIDE