[Misc] Clean up github tools files

  * Remove labeler configuration and actions: we don't want anymore to
    automatically labels PRs coming from Weblate. See the new strategy
for translation branches in https://forum.xwiki.org/t/translations-on-supported-branches/16041
  * Provide license and comment on backport configuration

(cherry picked from commit e877cc4e)

.github/labeler.yml

-backport stable-15.10.x:
-  - head-branch: ['^weblate']

.github/workflows/backport.yml

+## ---------------------------------------------------------------------------
+## See the NOTICE file distributed with this work for additional
+## information regarding copyright ownership.
+##
+## This is free software; you can redistribute it and/or modify it
+## under the terms of the GNU Lesser General Public License as
+## published by the Free Software Foundation; either version 2.1 of
+## the License, or (at your option) any later version.
+##
+## This software is distributed in the hope that it will be useful,
+## but WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+## Lesser General Public License for more details.
+##
+## You should have received a copy of the GNU Lesser General Public
+## License along with this software; if not, write to the Free
+## Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
+## 02110-1301 USA, or see the FSF site: http://www.fsf.org.
+## ---------------------------------------------------------------------------
+## Automatically backport merged pull requests that are labelled with a label containing "backport"
+## using https://github.com/tibdex/backport.
 name: Backport
 on:
   pull_request_target:

.github/workflows/labeler.yml

-name: "Pull Request Labeler"
-on:
-  - pull_request_target
-
-jobs:
-  labeler:
-    permissions:
-      contents: read
-      pull-requests: write
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/labeler@v5
\ No newline at end of file