storage: reconsider default for `--cache`

[nicktrav]

The default is currently 128 MiB. As the flag is not required, users often omit it, which can result in unexpected performance due to the very small cache size.

Some alternatives:

• Switch the default to use a percentage of available system memory (e.g. 25%).
• Make the flag required, which forces users to set it to a sensible value. The advantage of this approach is that it would also allow us to remove a bunch of code that warns about the default value.

We should also update our public documentation to reflect this change.

Jira issue: CRDB-40856