8.20.0+0.20.0

CHANGES:

• [serapi] (!) support for Coq 8.20, thanks to all the developers
  that contributed compatibility patches.
• [serlib] Support micromega_core plugin (@ejgallego)
• [serlib] Compat with ppx_deriving 6 (@ejgallego)
• [serlib] Move serlib sources to coq-lsp (@ejgallego, #409)
• [general] Drop support for OCaml 4.09-4.11 (@ejgallego, #409)
• [serlib] Embed serlib from coq-lsp as to help 8.20 release (@ejgallego)
• [meta] Add license info for sertop.el (@ejgallego, @SnarkBoojum, fixes #411)