Refactor DashboardWrapper, Page demos to use Masthead (over PageHeader)

[kmcfaul]

DashboardWrapper and full page demos that still include PageHeader should be refactored to use the Masthead component instead.

[kmcfaul]

Follow up to #6170

[nicolethoen]

Also, there is an advanced core demo introduced in this PR that would likely demonstrate the same things as this wrapper
patternfly/patternfly#4211