[Form components] Ensure all placeholder text meets WCAG accessibility color contrast requirements

[cee-chen]

Originally discussed by @1Copenut and @cee-chen in https://github.com/elastic/eui/issues/6630#issuecomment-1452720380

I think we need to take the time later to evaluate our placeholder text color contrast across the board / across all form/input elements, not just this one. IIRC WCAG allows lower contrast ratios for disabled elements... but apparently not for placeholder text (https://www.w3.org/WAI/WCAG21/Understanding/contrast-minimum.html) 😕

Unfortunately, our current form/input component native placeholder text and our components that mock pseudo placeholder text (EuiComboBox, EuiSuperSelect) likely do not meet color contrast minimums. We should address this across the board in all components with placeholder text if possible.

[github-actions[bot]]

👋 Hi there - this issue hasn't had any activity in 6 months. If the EUI team has not explicitly expressed that this is something on our roadmap, it's unlikely that we'll pick this issue up. We would sincerely appreciate a PR/community contribution if this is something that matters to you! If not, and there is no further activity on this issue for another 6 months (i.e. it's stale for over a year), the issue will be auto-closed.

[github-actions[bot]]

👋 Hi there - this issue hasn't had any activity in 6 months. If the EUI team has not explicitly expressed that this is something on our roadmap, it's unlikely that we'll pick this issue up. We would sincerely appreciate a PR/community contribution if this is something that matters to you! If not, and there is no further activity on this issue for another 6 months (i.e. it's stale for over a year), the issue will be auto-closed.