Don't add date to SDK changelog entry headers

[parlough]

Problem

Currently, once a release is cut (or sometime after the fact), the date is added to the header element for that release. For example, ## 3.5.0 became ## 3.5.0 - 2024-08-06. This breaks existing links to the pre-existing header and also prevents automatic determination of the fragment for a specific release. Specifically, this has caused broken links on dart.dev a few times. More often if the date is updated on the main branch later on.

Solution

As GitHub doesn't have the way of specifying a specific fragment for a header, instead of modifying the header, place the date separately. There are many options, but something simple like the following could work.

## 3.5.0

**Released on:** 2024-08-06

### Language

// ....

@athomas @itsjustkevin

[FMorschel]

@parlough I believe this doc for CPs should be updated too https://github.com/dart-lang/sdk/blob/main/docs/Cherry-picks-to-a-release-channel.md#changelog