Bertrand release for Coq 8.12

Release compatible with Coq 8.8 to 8.12, featuring the following changes:

• port to Coq 8.12
• switch from the deprecated omega tactic to lia

Bertrand release for Coq 8.11

Release compatible with Coq 8.8 to 8.11, featuring the following changes:

• port to Coq 8.11
• add metadata
• improve build scripts
• support optional building with Dune