build(deps): bump DavidAnson/markdownlint-cli2-action from 17 to 18

Bumps [DavidAnson/markdownlint-cli2-action](https://github.com/davidanson/markdownlint-cli2-action) from 17 to 18.
- [Release notes](https://github.com/davidanson/markdownlint-cli2-action/releases)
- [Commits](DavidAnson/markdownlint-cli2-action@v17...v18)

---
updated-dependencies:
- dependency-name: DavidAnson/markdownlint-cli2-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/github-cxx-qt-tests.yml

@@ -50,7 +50,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v4
-      - uses: DavidAnson/markdownlint-cli2-action@v17
+      - uses: DavidAnson/markdownlint-cli2-action@v18
         with:
           globs: '**/*.md'