Dashboard: Remove old panel code and leave only new panel design

[polibb]

What is this feature?

• Remove the newPanelChromeUI feature flag that we have already enabled by default.
• Remove old code that the old panel header was using: PanelHeader, PanelHeaderLoadingIndicator, PanelHeaderMenuTrigger
• Leave whichever components from the old panel are used elsewhere, but move them to the places they are used, instead of leaving them where it's not relevant for them anymore:
  PanelHeaderMenuItem is only used by PageHeader, hence moved there
  PanelHeaderCorner is only used by PanelEditorTableView, hence moved there
• Removed components that had corresponding components with the word "New" in them, as the latter will be the only ones now: PanelHeaderMenuWrapper -> PanelHeaderMenuWrapperNew, PanelHeaderMenu -> PanelHeaderMenuNew

Why do we need this feature?

Because we are not supporting the old panel setup and should have only the new one in place.

Who is this feature for?

Grafana users.

Which issue(s) does this PR fix?:

Fixes #70790

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

Release notice breaking change

Removed all components for the old panel header design.

[axelavargas]

Thank you for checking it out, @axelavargas! I did notice that we haven't updated the PanelEditorTableView, but thought this was our plan for some reason - that the table has a life of its own 😄. Anyway, I will look into it to update it. Could we do it in a separate following PR? If yes, I'll add it to the leftover tasks: #70790. The breaking change is the removed FF, is that not how it usually goes with removing feature flags?

Yes, we can do it in a separate PR :).