Increase default max value for slasher database size to 512

[jimmygchen]

Description

The default max DB size is specified here:

lighthouse/slasher/src/config.rs

Line 14 in b0c374c

pub const DEFAULT_MAX_DB_SIZE: usize = 256 * 1024; // 256 GiB

This is now insufficient and needs to be bumped to 512 Gb.

michael on discord:

256 is actually too small these days, you need to bump it to ~512ish

In addition to the default value, the docs / book needs to be updated as well.

[chong-he]

I have updated the doc since I have a PR on updating the doc now:
e6890f1

[chong-he]

Also update the default max db size:
1c72524

[chong-he]

Closed by #5934