build: bump peaceiris/actions-mdbook from 1 to 2 (openmls#1552)

Bumps [peaceiris/actions-mdbook](https://github.com/peaceiris/actions-mdbook) from 1 to 2.
- [Release notes](https://github.com/peaceiris/actions-mdbook/releases)
- [Changelog](https://github.com/peaceiris/actions-mdbook/blob/main/CHANGELOG.md)
- [Commits](peaceiris/actions-mdbook@v1...v2)

---
updated-dependencies:
- dependency-name: peaceiris/actions-mdbook
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: raphaelrobert <raphaelrobert@users.noreply.github.com>

.github/workflows/gh-pages.yml

@@ -19,7 +19,7 @@ jobs:
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
       - name: Setup mdBook
-        uses: peaceiris/actions-mdbook@v1
+        uses: peaceiris/actions-mdbook@v2
         with:
           mdbook-version: 'latest'
       - name: Build docs