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/button/test/a11y/button.e2e.ts

@@ -2,7 +2,7 @@ import AxeBuilder from '@axe-core/playwright';
 import { expect } from '@playwright/test';
 import { configs, test } from '@utils/test/playwright';
 
-configs({ directions: ['ltr'], themes: ['light', 'dark'] }).forEach(({ title, config }) => {
+configs({ directions: ['ltr'], palettes: ['light', 'dark'] }).forEach(({ title, config }) => {
   test.describe(title('button: a11y for ion-color()'), () => {
     test('should not have accessibility violations', async ({ page }) => {
       await page.setContent(
@@ -52,7 +52,7 @@ configs({ directions: ['ltr'], themes: ['light', 'dark'] }).forEach(({ title, co
 /**
  * Only ios mode uses ion-color() for the activated button state
  */
-configs({ directions: ['ltr'], modes: ['ios'], themes: ['light', 'dark'] }).forEach(({ title, config }) => {
+configs({ directions: ['ltr'], modes: ['ios'], palettes: ['light', 'dark'] }).forEach(({ title, config }) => {
   test.describe(title('button: ios contrast'), () => {
     test('activated state should not have accessibility violations', async ({ page }) => {
       await page.setContent(