Documentation of "Memory saving" (APCu section) outdated

[tobias47n9e]

This is:

- [x] an outdated part of the documentation
- [ ] a bug report
- [ ] a feature request
- [ ] **not** a usage question (ask them on https://stackoverflow.com/questions/tagged/phpspreadsheet or https://gitter.im/PHPOffice/PhpSpreadsheet)

What is the expected behavior?

Documentation should not recommend unmaintained packages.

What is the current behavior?

Documentation recommends cache/simple-cache-bridge and cache/apcu-adapter, which both don't seem to be well maintained.

Link: https://phpspreadsheet.readthedocs.io/en/latest/topics/memory_saving/#memory-saving

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I would send a better snippet, but not sure what the best recommendation would be.