Porting away from PlasmaExtras.Heading

With frameworks/kirigami!1120 (merged) and https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/801, we don't have to use the PlasmaExtras version of anything in Kirigami as long as it doesn't internally use any components that use SVG styling. At this point that's just PlasmaExtras.Heading, so let's port away from it.

Merge Requests and commits for this work:

• https://invent.kde.org/frameworks/plasma-framework/-/commit/bb7cee60554d86aeca2653b0e5a730925b76af3b
• https://invent.kde.org/frameworks/plasma-framework/-/commit/191288e4f26ecb54ce74b003370ef426acbed091
• !2999 (merged)
• plasma-desktop!1574 (merged)
• kdeplasma-addons@051ee881
• plasma-sdk@20accc05
• kscreenlocker!158 (merged)
• kscreen@45cee20f
• kwin@fb6edd72
• powerdevil@80a8f8bd

To be able to use Kirigami components that are the same as their PlasmaExtras versions except for the fact that they use SVG-themed components (such as PlaceholderMessage), we will need a bit more work; that's TBD.