Clean up "churn limit" code

[michaelsproul]

Description

With EIP-8061 we are getting yet more churn limit functions, and it's becoming confusing which are which. E.g. get_activation_churn_limit was defined by us previously but is now a spec function.

We should do an audit of all churn-limit related functions on the beacon state and in single-pass epoch processing so we can avoid bugs. I think we should gate all the old functions by a fork check (and error) so that they don't get called accidentally in the wrong place.

[michaelsproul]

Example of something that is already wonky in unstable, we are calculating validator-denominated churn limits, which are only legal pre-Electra, for every fork:

lighthouse/consensus/state_processing/src/per_epoch_processing/single_pass.rs

Lines 157 to 158 in 176cce5

	let churn_limit = state.get_validator_churn_limit(spec)?;
	let activation_churn_limit = state.get_activation_churn_limit(spec)?;

If we ban calling those functions post-Electra then this needs refactoring to make those optional/fork-scoped