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 (!7476 (merged)).

• !7096 (merged) - tests only
• !7541 (merged)
• !7571 (merged)
• !7606 (merged)
• !7656 (merged)

The list was generated using the following commands:

$ git log --reverse --merges --full-history 0b861fd0..69656b8a -- 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.