Clean up outdated mention of "max quorum nesting level = 2"

[jayz22]

Description

The incorrect maximum nesting level of 2 are still being mentioned at least in a couple places, the XDR definitions of SCPQuorumSet and the example cfg file.

It was once a legacy requirement which later was invalidated with automatic quorum generation based on quality level, and was invalidated once again when CRITICAL quality was introduced. So the correct value should be 4 (as defined in

stellar-core/src/scp/QuorumSetUtils.cpp

Line 16 in 0579fd9

uint32 const MAXIMUM_QUORUM_NESTING_LEVEL = 4;

)

This issue tracks cleanup of any outdated mention of the wrong nesting level (this includes any code comments in the XDR, stellar-core code base including example cfg, and possibly the docs).