epub2 deprecation

[HelenaSabel]

As discussed in the Stylesheets Group meeting on 2022-08-26:

• epub2 will stop being maintained
• the command teitoepub will have the same behaviour as teitoepub3
• the command teitoepub2 will be created (out of the current behaviour of teitoepub) with a warning saying that this format is not longer maintained (but fixing Errors reported by epubchecker in epub generated with 7.53.0 #568 before since it will be a minor change)