UI: improve A11Y remove redundant styling rules, update icon color

[tolkadot]

Closes #23455

What I did

Updated CSS for the accordion toggle icon within the accessibility pane.

• removed the muted, low contrast colour rule, colour is now the same as the text (#2E3438). This colour is cascaded to the icon from the body.
• removed redundant flex-box rules
• adjusted the alignment so the icon is centre aligned with the text.

CSS of the icon now looks as follows:

.css-1eq8i2q {
  display: inline-block;
  shape-rendering: inherit;
  fill: currentColor;
  height: 10px;
  width: 10px;
  min-width: 10px;
  margin-right: 10px;
  transition: transform 0.1s ease-in-out;
  vertical-align: inherit;
}

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• [ x ] unit tests
• integration tests
• end-to-end tests

Manual testing

To manually test this change, run a sandbox with the a11y add-on enabled.
Using the colour contrast checker[https://webaim.org/resources/contrastchecker/] verify that the arrow icon has sufficient colour contrast.

1. Run a sandbox for template, e.g. yarn task --task sandbox --start-from auto --template react-webpack/18-ts
2. Within the sandbox yarn add @storybook/addon-a11y --dev
3. add '@storybook/addon-a11y', to the addons array in storybook/sandbox/react-webpack-18-ts/.storybook/main.ts
4. yarn storybook
5. Open Storybook in your browser
6. Access any story
7. In the Addons panel check the accessibility addon panel
8. Verify the accordion toggle icon has sufficient colour contrast

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli/src/sandbox-templates.ts

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>

[tolkadot]

If possible can this be labeled with hacktoberfest & hacktoberfest-accepted tnx.

[cdedreuille]

LGTM!