Remove support for CodeQL 2.12.5 and earlier #1520

Job	Run time
Resolve environment (ubuntu-latest, stable-v2.13.4)	35s
Resolve environment (macos-latest, stable-v2.13.4)	1m 34s
Resolve environment (windows-latest, stable-v2.13.4)	2m 27s
Resolve environment (ubuntu-latest, default)	20s
Resolve environment (macos-latest, default)	2m 16s
Resolve environment (windows-latest, default)	1m 54s
Resolve environment (ubuntu-latest, latest)	27s
Resolve environment (macos-latest, latest)	1m 57s
Resolve environment (windows-latest, latest)	2m 29s
Resolve environment (ubuntu-latest, nightly-latest)	56s
Resolve environment (macos-latest, nightly-latest)	2m 25s
Resolve environment (windows-latest, nightly-latest)	2m 42s
	20m 2s

Provide feedback