[Diagnostics] Eliminate educational notes in favor of diagnostic groups

[DougGregor]

We've been converging the implementations of educational notes and diagnostic groups, where both provide category information in diagnostics (e.g., [#StrictMemorySafety]) and corresponding short-form documentation files. The diagnostic group model is more useful in a few ways:

• It provides warnings-as-errors control for warnings in the group
• It is easier to associate a diagnostic with a group with GROUPED_ERROR/GROUPED_WARNING than it is to have a separate diagnostic ID -> mapping.
• It is easier to see our progress on diagnostic-group coverage
• It provides an easy name to use for diagnostic purposes.

Collapse the educational-notes infrastructure into diagnostic groups, migrating all of the existing educational notes into new groups. Simplify the code paths that dealt with multiple educational notes to have a single, possibly-missing "category documentation URL", which is how we're treating this.

[DougGregor]

@swift-ci please smoke test

[DougGregor]

Huh.

FAIL: Swift(macosx-x86_64) :: Python/python_lint.swift (6074 of 18893)
******************** TEST 'Swift(macosx-x86_64) :: Python/python_lint.swift' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
./utils/dev-scripts/scurve_printer.py:17:5: F824 `global FIELDS` is unused: name is never assigned in scope
./utils/swift_build_sdk_interfaces.py:242:5: F824 `global args` is unused: name is never assigned in scope
./utils/swift_build_sdk_interfaces.py:242:5: F824 `global shared_output_lock` is unused: name is never assigned in scope
./utils/symbolicate-linux-fatal:78:5: F824 `global known_memmap` is unused: name is never assigned in scope
./utils/symbolicate-linux-fatal:93:5: F824 `global lldb_target` is unused: name is never assigned in scope
./utils/symbolicate-linux-fatal:117:5: F824 `global lldb_target` is unused: name is never assigned in scope
./utils/symbolicate-linux-fatal:118:5: F824 `global known_memmap` is unused: name is never assigned in scope
./utils/type-layout-fuzzer.py:46:9: F824 `global typesDefined` is unused: name is never assigned in scope
./utils/type-layout-fuzzer.py:88:5: F824 `global classesDefined` is unused: name is never assigned in scope

[DougGregor]

This ended up getting merged as part of #80392