Release v0.9.0 (sktime#1682)

* release changes

* release changes

* release changes

* release changes

* release changes

* release changes

* release changes

* release changes

* remove deprecated functionality

* remove deprecated functionality

* remove deprecated functionality

* update all contributors

* update all contributors

* update all contributors

Release v0.8.1

Release v0.8.0 (sktime#1431)

* 0.8.0 changelog

* clarification

* clarification

* updated forecaster multivariate comment

* Update changelog, bump version

Co-authored-by: mloning <[email protected]>

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

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