Fix changelog tag detection logic (second attempt)

Contributes to #437616 (closed) and #441804 (closed)

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