fix(input): insufficient colour contrast in input placeholder

[nuria1110]

fix #6587

Proposed behaviour

Placeholder in text-based inputs pass colour contrast accessibility tests.

Current behaviour

Placeholders in text-based inputs fail colour contrast accessibility tests.

Checklist

• [x] Commits follow our style guide
• [x] Related issues linked in commit messages if required
• [ ] Screenshots are included in the PR if useful
• [ ] All themes are supported if required
• [x] Unit tests added or updated if required
• [ ] Playwright automation tests added or updated if required
• [ ] Storybook added or updated if required
• [ ] Translations added or updated (including creating or amending translation keys table in storybook) if required
• [ ] Typescript d.ts file added or updated if required
• [ ] Related docs have been updated if required

QA

• [x] Tested in provided StackBlitz sandbox/Storybook
• [ ] Add new Playwright test coverage if required
• [ ] Carbon implementation matches Design System/designs
• [ ] UI Tests GitHub check reviewed if required

Additional context

readOnly style has been removed as the text colour does not change from colorsUtilityYin090.

Testing instructions