Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[x] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

This PR is a follow-up to #20155.

In this PR, I've added uniqueItems: true to the schema for the exceptMethods option of the class-methods-use-this rule.

Given the rule's description and implementation, preventing duplicate exception names seems valid and safe.

• https://eslint.org/docs/latest/rules/class-methods-use-this#exceptmethods

Currently, adding the following test case to the valid tests does not produce any errors:

		{
			code: "class A { foo() {this} bar() {} }",
			options: [{ exceptMethods: ["bar", "bar"] }], // Duplicated.
			languageOptions: { ecmaVersion: 6 },
		},

Is there anything you'd like reviewers to focus on?

N/A