No more separate v1 and v2 changelog. Let's use v2 changelog only named as CHANGELOG.md and CHANGELOG-alpha.md

Because we used to publish a v1 and v2 of the CDK at the same time, we had two different files for the change log.
Now that we only use v2, this is not relevant anymore and we should drop the `v2` suffix and use the "nice" name as the source for all v2 changes. We also don't need to track the v1 changes on the main branch.

Additionally the file is getting unwieldy large. We should find a solution for that as well.

Start by locating the existing v1 and v2 changelog files and their references in the repository. Determine how v2 changes should use CHANGELOG.md and CHANGELOG-alpha.md, and decide how the oversized changelog should be managed; done means v1 tracking is removed from the main branch and the v2 naming is consistently used.

Written by the indexing model from the issue text.