Fix changelog tag detection logic

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/437616

**Problem**

`ChangelogTagFinder` expected user to provide a version in semver
format (without v at the beginning). However, DEFAULT_TAG_REGEX allows
to use `v` at the start of the string.

As a result, `VersionSorter` cannot correctly sort versions and user
receives an error that the version is incorrect.

**Solution**

* Convert user specified version to format that `VersionSorter`
supports.
* Add test to verify that versions like `v1.0.0` are accepted.
* Update Changelog API documentation

Changelog: fixed