Update CHANGELOG with 'final' and 'var' modifier breaking changes.

Clarify in the 3.13 section to include the breakages that come with
the new declaring parameters. We're no longer able to use 'final' nor
'var' as modifiers for function parameters.

Also updated the primary constructor spec link to point straight to the
primary constructor spec instead of the forwarding stub.

Bug: https://github.com/dart-lang/sdk/issues/64151
Change-Id: I5cbf0cb721cdea76963a9c39c906a9528870f577
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/542502
Commit-Queue: Kallen Tu <kallentu@google.com>
Reviewed-by: Bob Nystrom <rnystrom@google.com>