test: remove TS 6 RC from test matrix (#907)

<!-- 👋 Hi, thanks for sending a PR to ts-api-utils! 💖.
Please fill out all fields below and make sure each item is true and [x]
checked.
Otherwise we may not be able to review your PR. -->

## PR Checklist

- [ ] Addresses an existing open issue: fixes #000
- [ ] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/ts-api-utils/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [ ] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/ts-api-utils/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

Since TS 6 is "latest" now.

Author: kirkwaiblinger

.github/workflows/test.yml

@@ -27,10 +27,8 @@ jobs:
           - latest
         ts_version:
           - 5.5.4
+          - 5.9.3
           - latest
-          # TS 6 rc. Once TS 6 is released, this can be removed, since it will
-          # be superseded by `latest`
-          - rc
 
 name: Test