doc/next: add release notes for runtime and sync

doc/next: add release notes for runtime and sync

Also, move the weak package heading to the end; it currently overlaps
with 1-osroot.md in the sort order.

For #68545.

Change-Id: Ia26ae511eaab7c8762e42d1593ac2197d25bbadb
Reviewed-on: https://go-review.googlesource.com/c/go/+/633599
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>