Delete defunct GHA workflows

[Jake Bailey (jakebailey)]

• We don't need LKG, as we don't check in the final compiler to the repo anymore.
• pr-modified-files only works if we run CI on every PR regardless of who opened it, which we don't do anymore. I will do "generated DOM file" check in the repos automation repo later.
• We don't need "release branch artifact"; I don't think we ever used this.

I'm not deleting the other ones yet, as they will be useful for bumping, cutting, etc, when the time comes.

[Copilot]

Pull request overview

Removes three obsolete, already-disabled GitHub Actions workflows.

Changes:

• Removes legacy LKG and release artifact workflows.
• Removes obsolete PR modified-files automation.
• Retains disabled workflows needed for future release management.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
.github/disabled-workflows/release-branch-artifact.yaml	Removes unused release artifact packaging.
.github/disabled-workflows/pr-modified-files.yml	Removes obsolete PR file checks.
.github/disabled-workflows/lkg.yml	Removes defunct LKG automation.

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