BN: Move --db-backup-output-dir as a deprecated flag.

[nalepae]

What type of PR is this?
Bug fix

What does this PR do? Why is it needed?
No functional change.

New --help output:

...
interop OPTIONS:
   --genesis-state value           Load a genesis state from ssz file. Testnet genesis files can be found in the eth2-clients/eth2-testnets repository on github.
   --interop-genesis-time value    Specify the genesis time for interop genesis state generation. Must be used with --interop-num-validators (default: 0)
   --interop-num-validators value  Specify number of genesis validators to generate for interop. Must be used with --interop-genesis-time (default: 0)

deprecated OPTIONS:
   --db-backup-output-dir value  Output directory for db backups.

I do not remove this flag not to break compatibility.

An other PR in documentation will remove the section for BN DB backup.

Which issues(s) does this PR fix?
Fixes #13449