Add 2.7.0 release notes

[IAlibay]

Nearly forgot this, we probably should automate it somehow.

@fiona-naughton - this is an extra step in the release process I forgot to go over.

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📚 Documentation preview 📚: https://mdanalysisuserguide--353.org.readthedocs.build/en/353/

[IAlibay]

Odd it's picking up mda 2.7.0-dev0 not 2.8.0-dev0

[IAlibay]

🤦🏽‍♂️ 3 months ago...