[ci][web] Run tests in canvaskit mode. (#6879)

Configures the flutter/packages CI to run web tests using the `canvaskit` renderer, which is the new default for web apps.

It also does some minor changes:

* Skips tests on the web for `package:palette_generator` (cleanup issue created below)
  * Makes the `palette_generator` example run on the web. This was used to manually verify that the package still works as expected with canvaskit.
* Tweaks the README of the `pointer_interceptor` example apps, so they match what's happening in there.

## Issues

* Fixes https://github.com/flutter/flutter/issues/143543

Co-authored-by: David Iglesias <ditman@gmail.com>

script/tool/lib/src/drive_examples_command.dart

@@ -119,7 +119,7 @@ class DriveExamplesCommand extends PackageLoopingCommand {
           'web-server',
           '--web-port=7357',
           '--browser-name=chrome',
-          '--web-renderer=html',
+          '--web-renderer=canvaskit',
           if (platform.environment.containsKey('CHROME_EXECUTABLE'))
             '--chrome-binary=${platform.environment['CHROME_EXECUTABLE']}',
         ],