`PrecedenceGroupRelationSyntax.higherThanOrLowerThan` should be named `higherThanOrLowerThanKeyword`

[AnthonyLatsis]

Description

The current name is inconsistent with PrecedenceGroupAssignmentSyntax.assignmentKeyword and PrecedenceGroupAssociativitySyntax.associativityKeyword.

Steps to Reproduce

No response

[ahoppen]

Tracked in Apple’s issue tracker as rdar://110482259