CHANGELOG.md: move the date out of headings

For insider version, the pre-release will be kept as part of the
heading.

For #3531

Change-Id: Id9cffb88ba93a33ceba934e078f49b8ce1821f68
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/613696
kokoro-CI: kokoro <noreply+kokoro@google.com>
Commit-Queue: Hongxiang Jiang <hxjiang@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

Author: h9jiang