feat(VExpansionPanelTitle): add hover prop to disable hover effect

[mixelburg]

Summary

Closes #21615

There was no built-in prop to disable the hover overlay effect on v-expansion-panels; the only option was a CSS workaround:

.v-expansion-panel-title:hover > .v-expansion-panel-title__overlay {
  --v-hover-opacity: 0;
}

This PR adds a hover boolean prop (default: true) to both VExpansionPanelTitle and VExpansionPanels.

Usage

<!-- Individual panel title -->
<v-expansion-panel-title :hover="false">Title</v-expansion-panel-title>

<!-- All panels in a group -->
<v-expansion-panels :hover="false">
  <v-expansion-panel>...</v-expansion-panel>
</v-expansion-panels>

Changes

• VExpansionPanelTitle.tsx — adds hover: { type: Boolean, default: true } prop and applies the v-expansion-panel-title--no-hover modifier class when disabled
• VExpansionPanel.sass — adds CSS rule that forces overlay opacity to 0 when the modifier class is present
• VExpansionPanels.tsx — adds 'hover' to the pick() list and to provideDefaults so the value cascades to all child panels
• VExpansionPanelTitle.json — adds documentation for the new prop

[J-Sek]

• PRs adding new props need to target dev
• needs entry in new-in.json
• cannot use !important, should either add --hover class to "set" hover effect or make use of tools.layer('trumps') if there is no better way and we have to override the CSS from lower layers

I can see the changes in the dedicated tab, no need to repeat those - better to leave PR description for information that is not clear upon inspecting the code (like motivation behind some less-obvious of the changes).