Bump core from 99abec9 to bc68f84 (#1506)

Bumps [core](https://github.com/microsoft/typespec) from `99abec9` to
`bc68f84`.
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/microsoft/typespec/commit/bc68f8499a0ded302c8ddc47ef01d84bd49c8a84"><code>bc68f84</code></a>
Add release notes for version 0.60.0 (<a
href="https://redirect.github.com/microsoft/typespec/issues/4384">#4384</a>)</li>
<li>See full diff in <a
href="https://github.com/microsoft/typespec/compare/99abec93e9551e61a4393f6fd55e99db54efd3e9...bc68f8499a0ded302c8ddc47ef01d84bd49c8a84">compare
view</a></li>
</ul>
</details>
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