Core: Prepare for Migrate 4.0.0

[mgol]

Changes

1. Core: Remove support for jQuery 3.x

2. Core: Remove patches for breaking changes in jQuery 3.0.0 or older

The self-closed-tags patch remains, despite being a breaking change in jQuery
3.5.0, not 4.0.0. There are a few reasons for that:

1. It's an exception that a breaking change arrived in a non-major version bump.
   Some people may be upgrading from jQuery 3.4.0 or older and it's good to
   make it work for them.
2. The patch is disabled by default, so the concern of people on newer jQuery
   3.x upgrading to jQuery 4.x with Migrate 4.x getting patches restoring
   behavior from an version older than the pre-upgrade one does not exist.
3. This was a pretty big break, it may help people update if we still support
   it.

3. Core: Don't reimplement deprecated but not removed APIs

This will save space and avoid potential divergence from Core.

Also, simplify the deferred.pipe patch.

4. Core: Use console.info for deprecations without removals.

Also:

1. Group warnings.md by this type.
2. Rename jQuery.migrateWarnings to jQuery.migrateMessages.
3. Rename jQuery.migrateDeduplicateWarnings to
   jQuery.migrateDeduplicateMessages.
4. Adding "info" versions of some internal APIs.