CI: paths-ignore only ignores modifications in markdown files (#876) (#878)

Signed-off-by: eduponz <eduardoponz@eprosima.com>
(cherry picked from commit 491f70f)

Co-authored-by: Eduardo Ponz Segrelles <eduardoponz@eprosima.com>

Author: mergify[bot]

.github/workflows/ubuntu-ci.yml

@@ -49,8 +49,6 @@ on:
       - review_requested
     paths-ignore:
       - '**.md'
-      - '**.txt'
-      - '!**/CMakeLists.txt'
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}