ci: improve backport PR title and body

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.

Author: apricote

.github/workflows/backport.yml

@@ -23,4 +23,13 @@ jobs:
     steps:
       - uses: tibdex/backport@v2
         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 %>.
+