prep for 3.0 release

[braingram]

update changelog
remove CI tests for 2.15 branch
remove commented out description of 2.15 as a considered LTS in docs

[braingram]

The dkist and py312 jobs will not pass due to:

• dkist pinning gwcs 0.18.3 which warns about the bugs fixed in gwcs 0.19.0, as the dkist CI turns these into errors the job fails. See update warning ignore for dkist with 0.18.3 pin #1660 for passing dkist tests (by ignoring the gwcs warning)
• aiohttp (required by fsspec) has not yet released a python 3.12 compatible version. See TEST: Py312 #1651 for passing 3.12 tests (by installing the beta release of aiohttp 3.9.0).