Refactor ApprovalRules::MergeRequestRuleDestroyService to support v2 rules

Refactor ApprovalRules::MergeRequestRuleDestroyService to support v2 approval rules.

Used by REST API (DELETE) and GraphQL mutations for deleting approval rules. Currently only supports v1 rules. When v2_approval_rules FF is enabled, should delete v2 rules instead.

What

  • Check v2_approval_rules feature flag
  • Delete v2 rules when flag is enabled
  • Delete v1 rules when flag is disabled
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