Remove "default" referral program rule set

[lightwalker-eth]

In our Tuesday sync up call we discussed an update where the "default" referral program editions config would be removed and ENSApi would interpret an undefined CUSTOM_REFERRAL_PROGRAM_EDITIONS environment variable as a referral program editions config with no editions at all.

This will have the added benefit that for any ENSApi instance where CUSTOM_REFERRAL_PROGRAM_EDITIONS is undefined won't do any extra work in relation to loading referral data from ENSDb.

1. Since we're removing the "defaults" we should also remove the use of the word "custom".
2. Remove.
3. Make it more clear this is only related referral API requests and not all API requests.
4. Remove.
5. We should relax this constraint to allow for the array to be empty.
6. We should rename CUSTOM_REFERRAL_PROGRAM_EDITIONS to REFERRAL_PROGRAM_EDITIONS

[coderabbitai]

🔗 Related PRs

#1712 - rev-share-limit Referral Program Disqualification [merged]

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