Deprecate old notes script

This MR officially deprecates the old notes.js script which used to be the core implementation of our discussions on issues and merge requests.

A few years back this was replaced with a Vue-native implementation, but the old implementation still remains in use on the snippets pages and individual commit pages. Differentiating code that refers to the old implementation or the new one is difficult at the moment. Renaming this to deprecated_notes.js makes it easier to see where it is used and to target the places where it is used for refactoring.

This MR does the following things:

• Removes notes.js import from merge_request_tabs.js after confirming it is not needed or used there.
• Removes init_notes.js import from zen_mode_spec.js after confirming that it is not needed.
• Renames notes.js to deprecated_notes.js
• Renames init_notes.js to init_deprecated_notes.js

Once this is merged, it will be a bit easier to focus on refactoring bits and pieces of the old notes implementation, knowing with a bit more certainty what pages will actually be effected.

e.g. #241800 (closed)