feat!: Remove Paver

BREAKING CHANGE: Removes all remaining Paver commands including
`pavelib/prereqs.py:*` and `pavelib/assets.py:*`.

BREAKING CHANGE: Removes `./manage.py [lms|cms] compile_sass`, which
was just a wrapper around Paver commands.

BREAKING CHANGE: Removes paver.txt. Operators should install testing.txt
instead.

Part of: https://github.com/openedx/edx-platform/issues/34467

.github/workflows/unit-test-shards.json

@@ -255,15 +255,13 @@
   "common-with-lms": {
     "settings": "lms.envs.test",
     "paths": [
-      "common/djangoapps/",
-      "pavelib/"
+      "common/djangoapps/"
     ]
   },
   "common-with-cms": {
     "settings": "cms.envs.test",
     "paths": [
-      "common/djangoapps/",
-      "pavelib/"
+      "common/djangoapps/"
     ]
   },
   "xmodule-with-lms": {