Replace GET validators api calls with POST api where possible

[nflaig]

We might wanna start using the postStateValidators api instead of getStateValidators to avoid issues such as #6830.

This should be pretty safe to change this in CLI commands such as voluntary exit as users will likely interact with a Lodestar BN which implements the api

lodestar/packages/cli/src/cmds/validator/voluntaryExit.ts

Line 212 in 8b6ecc4

const res = await client.beacon.getStateValidators("head", {id: pubkeys});

We could also consider doing it in indicies store but need to ensure all other CL clients support the api (maybe switch after electra)

lodestar/packages/validator/src/services/indices.ts

Line 129 in 8b6ecc4

const res = await this.api.beacon.getStateValidators("head", {id: pubkeysHex});