ci: disable litbot checkgroup

[bhimrazy]

What does this PR do?

Ports the litbot checkgroup disable to release/stable.

Comments out the pytorch_lightning: lit GPU, lightning_fabric: lit GPU, and Benchmarks sections in .github/checkgroup.yml since LitBot is currently unavailable.

Without this, PRs targeting release/stable that touch src/lightning/pytorch/** are blocked: the probot reads checkgroup.yml from the base branch (release/stable) for security reasons on fork PRs, so those GPU/benchmark jobs are required but never triggered — causing a 70-minute timeout on required-jobs (see #21727).

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[github-actions]

⚡ Required checks status: All passing 🟢

No groups match the files changed in this PR.

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