Bump to v12.1

Going forward, an uneven minor version will be a development version, while an even minor will be
considered a stable version to be released. This is similar to what GHC does and was discussed in
#3113.

This bump should have happened after branching off v12.0.0, but obviously we didn't know about it
back then. This will happen immediately after branching off a new release from now on.

postgrest.cabal

@@ -1,5 +1,5 @@
 name:               postgrest
-version:            11.2.0
+version:            12.1
 synopsis:           REST API for any Postgres database
 description:        Reads the schema of a PostgreSQL database and creates RESTful routes
                     for tables, views, and functions, supporting all HTTP methods that security