chore(config): remove the deprecated configuration rocksdb.max_background_{compactions,flushes} and set default to -1

[yatriks]

This PR will:

• update the default values of rocksdb.max_background_compactions and rocksdb.max_background_flushes from 2 to -1
• unset them from kvrocks.conf and update comments to reflect RocksDB's decision logic for max background threads

NOTE: This doesn't make it so that rocksdb.max_background_compactions is unsettable, like rocksdb.max_background_flushes currently is. This may be desirable for people who do currently use the old overrides, but afaik it would lock people in to max_background_flushes = 2 - but that's already the case as far as I can tell.

I ran each of:

./x.py format (unrelated changes)
./x.py build --unittest
./x.py test cpp 

Tests all pass and formatting changes are unrelated.

This is effectively an addition to #1665
See #3395 for more background.

[yatriks]

Also I cannot recall if setting READONLY on the option makes it read-only for everything or just CLI...that would inform whether we update max_background_compactions to readonly. I can verify later.

[yatriks]

@git-hulk Sorry will need another review.
also set max_background_compactions to READONLY (which I checked doesn't preclude setting it from kvrocks.conf) for consistency. Feel free to revert this if it's problematic for a reason I did not foresee.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
60.0% Coverage on New Code
0.0% Duplication on New Code

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