Rename `approval_rule` method names to `specific_approval_rule` if it's referring to Specific Approval Rule

Problem

As we generalized the Approval Rules concept in !100382 (closed), some of the approval_rule occurrences actually refer Specific Approval Rule instead of Unified Approval Rule. In order to avoid confusion, all of these occurrences should be renamed as it's specific to Specific Approval Rule.

Proposal

Rename approval_rule to specific_approval_rule in Deployment Approval features.

Auto-generated

The following discussion from !100382 (closed) should be addressed:

• @ali-gitlab started a discussion: (+1 comment)

  Should we rename this to specific_approval_rules and make it private?