README: Update version reference

cmake: Bump project version to 2.6.1

README: Mark GnuTLS as optional

cmake: Prevent nested tarballs (or other archives) in the release

This happened dring the release of the version 2.5.2.

holidays: Update holidays using newest holidata.net data

Remove outdated holiday files that could not be updated.

Task Release 2.5.1

Task Release 2.5.0

Task Release 2.4.4

Task Release 2.4.3

Task Release 2.4.2