net/unison2.51: Update to 2.52.0

## Changes in 2.52.0

Released 2022-03-12

  * Feature negotiation, compatible wtih 2.51.
  * New archive format (independent of ocaml version, based on umarshal)
    Upgrade is automatic.
  * New wire protocol (independent of ocaml version, based on umarshal)
    New protocol is used if both sides are >= 2.52.0.
  * Compatibility with 2.48
  * Support for unix-domain sockets
  * Many bugfixes and minor improvements
  * ocaml compatibility is now >= 4.01
  * NEWS is now in NEWS.md and not in the manual

(gdt)