Doc: Update Alpha changelog

Context

Updating Alpha changelog to reflect protocol and environment changes merged into master before 23th of January.

The following MRs have touched src/proto_alpha/lib_protocol or src/lib_protocol_environment/sigs/v*/ since the last update of Alpha changelog (!7406 (merged)).

• !6921 (merged) - build and tests only
• !7341 (merged) - tests only
• !7360 (merged) - tests only
• !7433 (merged) - tests only
• !7438 (merged)

The list was generated using the following commands:

$ git log --reverse --merges --full-history 771d6d..067c78 -- src/proto_alpha/lib_protocol/ src/lib_protocol_environment/sigs/v*/ | grep 'See merge request' | sort

Manually testing the MR

Please read the description of changes.