CI - update GitHub actions to node20

[pavoljuhas]

Address deprecation warnings in CI workflow annotations at https://github.com/quantumlib/Cirq/actions/runs/7792545465

Namely update to

• actions/cache@v4 per https://github.com/actions/cache/releases/tag/v4.0.0
• actions/checkout@v4 per https://github.com/actions/checkout/blob/main/CHANGELOG.md
• actions/setup-node@v4 per https://github.com/actions/setup-node/releases/tag/v4.0.0
• actions/setup-python@v5 per https://github.com/actions/setup-python/releases/tag/v5.0.0

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (f2c6f3c) 97.81% compared to head (d3a413c) 97.81%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6445 +/- ## ======================================= Coverage 97.81% 97.81% ======================================= Files 1111 1111 Lines 97198 97198 ======================================= Hits 95078 95078 Misses 2120 2120 ```

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