Claiming compatibility to other RFCs/updates of RFC5545

[niccokunzmann]

These RFCs are updates to RFC5545. For each of them, it would be nice to check if we can claim that icalendar is compatible.

• RFC 6868
• RFC 7529 RFC 7529 - non gregorian RRULE #653
• RFC 7953
• RFC 7986
• RFC 9073
• RFC 9074
• RFC 9253

See also:

• https://www.rfc-editor.org/info/rfc5545

This issue can be closed when

• all of them are checked (possibly with tests)
• we say in the README if we are compatible or not - https://icalendar.readthedocs.io/en/latest/usage.html#compatibility

What are your thoughts on this?