[8.0.0] Bazel bootstrap: use MODULE.bazel to get the version (#24294)

This is more reliable than parsing CHANGELOG.md, which is not always the
latest.

This requires us to maintain the Bazel version number in MODULE.bazel,
which should be updated once a year on the master branch, but every time
we do a minor/patch release.

PiperOrigin-RevId: 695747413
Change-Id: I63d572445d8cb6a76a3e079c2ea6f083257216b0