Doc : update Alpha changelog

Context

Updating Alpha changelog to reflect protocol and environment changes merged into master before 20th of March.

The following MRs have touched src/proto_alpha/lib_protocol or src/lib_protocol_environment/sigs/v*/ since the last update of Alpha changelog (!8015 (merged)).

• !7721 (merged) - tests only
• !7739 (merged) - tests only
• !7771 (merged) - tests only
• !7778 (merged) - tests only
• !7921 (merged) - tests only
• !7927 (merged) - already included
• !7953 (merged)
• !8022 (merged) - tests only
• !8057 (merged) - tests only The list was generated using the following commands:

$ git log --reverse --merges --full-history c8278f5f..4fb4a85b -- 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.