Doc : update Alpha changelog

Context

Updating Alpha changelog to reflect protocol and environment changes merged into master before 30th 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 (!7671 (merged)).

• !7435 (merged) - tests only
• !7546 (merged)
• !7720 (merged)
• !7724 (merged)
• !7726 (merged)
• !7755 (merged) - tests only
• !7761 (merged) - benchmarks only
• !7774 (merged) - build system only

The list was generated using the following commands:

$ git log --reverse --merges --full-history 69656b8a..64e0f91b -- 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.