chore: Make Python 3.14 support official

[ValerianRey]

• Add 3.14 to the matrix of tested python versions in tests.yml
• Run build-deploy-docs.yml, release.yml, tests.yml:build-doc and tests.yml:mypy on 3.14
• Say torchjd is compatible with 3.14 in installation.md and add 3.14 classifier in pyproject.toml
• Suggest 3.14 in CONTRIBUTING.md
• Add changelog entry

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• The badge in README.md will be updated whenever we release. It's based on the classifiers in pyproject.toml.
• Status of python version can be found here. 3.14 was released on october 7, and 3.9 is end of life this month, so torchjd is now compatible with all current python versions (3.10 to 3.14).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
see 52 files with indirect coverage changes

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[PierreQuinton]

Should end of support of 3.9 be done here or in another PR? I think it may also allow some non-trivial positive change in typing.

[ValerianRey]

Should end of support of 3.9 be done here or in another PR? I think it may also allow some non-trivial positive change in typing.

lol we never supported 3.9