chore(playwright): migrate themes to palettes (#29148)

Issue number: N/A

---------

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## What is the current behavior?
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## What is the new behavior?
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- Migrates the test infrastructure to use `palettes` instead of `themes`

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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This **does not** change where the light/dark/high contrast styles live
or how they are consumed in the test infra. That work is done here:
https://github.com/ionic-team/ionic-framework/pull/29149

Co-authored-by: Liam DeBeasi <liamdebeasi@users.noreply.github.com>

core/src/components/label/test/a11y/label.e2e.ts

@@ -84,7 +84,7 @@ configs({ directions: ['ltr'] }).forEach(({ title, screenshot, config }) => {
   });
 });
 
-configs({ directions: ['ltr'], modes: ['md'], themes: ['light', 'dark'] }).forEach(({ title, config }) => {
+configs({ directions: ['ltr'], modes: ['md'], palettes: ['light', 'dark'] }).forEach(({ title, config }) => {
   test.describe(title('label: a11y for ion-color()'), () => {
     test('should not have accessibility violations when focused', async ({ page }) => {
       /**