docs: update chlog after 2.5.5 release

[Borda]

What does this PR do?

regular post release update https://github.com/Lightning-AI/pytorch-lightning/releases/tag/2.5.5

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📚 Documentation preview 📚: https://pytorch-lightning--21169.org.readthedocs.build/en/21169/

[github-actions]

⚡ Required checks status: All passing 🟢

No groups match the files changed in this PR.

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