Move the `snapcraft` configuration to `contrib`. (#12142)

* Move the `snapcraft` configuration to `contrib`.

We're happy for people to package this as a snap image if it's useful,
but we don't support or maintain it. I'd like to move the config to
`contrib` to reflect this state of affairs.

* Changelog

MANIFEST.in

@@ -52,5 +52,4 @@ prune contrib
 prune debian
 prune demo/etc
 prune docker
-prune snap
 prune stubs

changelog.d/12142.misc

+Move the snapcraft configuration file to `contrib`.
\ No newline at end of file