Remove remaining references to "approve" from External Status Checks API documentation

What does this MR do?

To avoid confusion with Merge Request approvals, and to maintain a distinction between the value of human and bot judgments, we do not use the term "approve" or "approval" in relation to External Status Checks. This updates the API documentation page to remove the term from a few locations, because it was briefly in use.

Relevant threads about not using "approve|approval":

• #327634 (comment 557105671)
• #329995 (comment 568105675)
• !59137 (comment 568008390)

Note: The preferred term for External Status Check "approval" is "pass" (and the opposite term is "fail", to be introduced under #329636 (closed)). Both of these terms appear in existing documentation.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.