Release v0.8.0 (sktime#1431)

* 0.8.0 changelog

* clarification

* clarification

* updated forecaster multivariate comment

* Update changelog, bump version

Co-authored-by: mloning <markus.loning.17@ucl.ac.uk>

release 0.7.0 version up and changelog update

Release v0.6.1

Release v0.6.0

Release v0.5.3

Release v0.5.2

Release v0.5.1

Update GitHub Action workflows

Update release maker

Update PR template