ci: make NOTICE.txt drift just a warning

[JoshMock]

Getting Renovate to update NOTICE.txt correctly is proving to be a big pain. Instead, let's make NOTICE.txt drift a warning, not a hard requirement.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	1		0	0	0.05s
✅ COPYPASTE	jscpd	yes		no	no	9.32s
✅ REPOSITORY	gitleaks	yes		no	no	57.02s
✅ REPOSITORY	git_diff	yes		no	no	0.56s
✅ REPOSITORY	secretlint	yes		no	no	30.64s
✅ REPOSITORY	trivy	yes		no	no	18.44s
✅ YAML	yamllint	1		0	0	0.7s

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