Document that the libpst VCS has been converted from Mercurial to Git

The conversion was done using hg-fast-export and git-filter-repo.

The new maintainer has a preference for Git over Mercurial.

Git has a larger user base and more popular forges such as GitHub.

See-also: https://git-scm.com/
See-also: https://github.com/
See-also: https://github.com/frej/fast-export
See-also: https://github.com/newren/git-filter-repo