Deprecate iconType and iconProps from OuiPageHeader

[KrooshalUX]

As including an icon in our page header is not part of our guidance moving forward, we should deprecate the use of iconType and iconProps from OuiPageHeader, as well as update the corresponding OUI documentation page (remove the props from the props tab, and in any of the playground areas it may still be visible).

Thanks to @joshuarrrr for the suggestion!

          @KrooshalUX Do we want a follow-up issue to actually remove/deprecate the `<OuiPageHeader>` `iconType` prop?

Originally posted by @joshuarrrr in #625 (comment)

[curq]

Can I take this issue please?

[BSFishy]

For reference, this work should be done in 2 PR's:

1. Deprecate the prop by adding a doc comment and a console warning
2. Actually remove the prop

This will make sure we can backport the deprecation