Use maxCommandForkCount gamerule for limiting execute

[amyavi]

This PR replaces the execute as/at limiter with the maxCommandForkCount gamerule (introduced in 1.20.3), which does the same thing the execute limiter intended to do.
The limit for the gamerule is the same as the config's maxEntitiesPerWorld, which is the
theoretical limit for the old limiter (/execute as @e ...).

Since the maxCommandForkCount gamerule also applies to nested execute commands, I removed the block for execute-in-execute aswell.
The code for the gamerule limiter was getting a bit messy, so I refactored that. The gamerule limits are now applied to all of the worlds on startup, because Minecraft's default for maxCommandForkCount is 65535, which is way above ours.