v2.3: Update CHANGELOG.md for v3.0 (backport of #6424)

[mergify]

Problem

We are updating branches so update the links in changelog

---

This is an automatic backport of pull request #6424 done by Mergify.

[brooksprumo]

I don't think we want v3.0 info in the v2.3 changelog.

[steviez]

I don't think we want v3.0 info in the v2.3 changelog.

I agree; however, we had that remnant in all of our previous changelogs. Ie, if you check out the v2.2 one, you'll see mention of v2.3 there as well:
https://github.com/anza-xyz/agave/blob/v2.2/CHANGELOG.md

We just had a quick hash out about this; the idea is to:

• Land this PR (for sake of consistency with previous releases)
• Add a follow-on PR that strips the v3.0 and the version-->release mapping. Ie, looking at the v2.2 changelog, this will be removed:

Release channels have their own copy of this changelog:
* [edge - v2.3](#edge-channel)
* [beta - v2.2](https://github.com/anza-xyz/agave/blob/v2.2/CHANGELOG.md)
* [stable - v2.1](https://github.com/anza-xyz/agave/blob/v2.1/CHANGELOG.md)

<a name="edge-channel"></a>
## 2.3.0 - Unreleased

[brooksprumo]

We just had a quick hash out about this; the idea is to:

Gotcha, sounds good. Thanks for the info.