Various fixups:

- Use u32 for correct parameter formatting in pageserver mgmt API
  client
- Use a correctly formatted PgVersionId (not: PgMajorVersion) in
  test_branching.py
- Add some documentation on postgres_versioninfo::PgVersionId

test_runner/regress/test_branching.py

@@ -172,7 +172,7 @@ def test_cannot_create_endpoint_on_non_uploaded_timeline(neon_env_builder: NeonE
             env.initial_tenant,
             env.initial_timeline,
             MembershipConfiguration(generation=1, members=[sk.safekeeper_id()], new_members=None),
-            int(env.pg_version),
+            int(env.pg_version) * 10000,
             Lsn(0),
             None,
         )