Hydras & Co. 0.6

This release has been tested for compatibility with Coq 8.13-8.15 and MathComp 1.12-1.14.
The gaia-hydras package requires Coq 8.14 or later.

Release highlights

• Build a bridge with the Gaia library by @Casteran in #112, #115, #117 and #120
• Use LibHyps by @Casteran in #119

Other changes

• Move definition of gaia-hydras package to nixpkgs. by @Zimmi48 in #98, #99 and #101
• Update README.md on gaia and reference JFLA 2022 paper by @palmskog in #104 and #107
• Fix scheme issue by @Casteran in #116
• Test compatibility with Coq 8.15 by @palmskog in #119

Full Changelog: v0.5...v0.6