Components: Fix placeholder color inconsistency across SearchControl, TextControl, Inputcontrol

[im3dabasia]

Closes: #64666

What?

This PR ensures a consistent placeholder color across the InputControl, SearchControl, and TextControl components by defining the color explicitly.

Why?

To address the issue where placeholder color varied depending on the applied styles.

How?

The placeholder color has been defined at the component level to avoid inconsistencies and improve accessibility.

Testing Instructions

1. Test the InputControl, SearchControl, and TextControl components in the storybook
2. Ensure the placeholder color is consistent and meets the contrast ratio requirements.

Component	Foreground Color	Background Color
InputControl	#1e1e1e9e (dark gray 62% opacity)	#ffffff (white)
SearchControl	#1e1e1e9e (dark gray 62% opacity)	#f0f0f0 (light gray)
TextControl	#1e1e1e9e (dark gray 62% opacity)	#ffffff (white)

Screenshots

SearchControl

• Their are talks around removing the background color to make the input box consistent with other input boxes Unify the search fields #56388 (comment)

Before	After

TextControl

Before	After

InputControl

Before	After

[im3dabasia]

• This PR aims to standardize the placeholder colors for the three components mentioned above. There has been an ongoing discussion about potentially removing the light gray color from the SearchControl, as noted in this issue comment.

• I am happy to take on the task of working on removing the color, if needed.

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Co-authored-by: im3dabasia <[email protected]>
Co-authored-by: Mamaduka <[email protected]>
Co-authored-by: afercia <[email protected]>
Co-authored-by: t-hamano <[email protected]>

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[im3dabasia]

Hey @Mamaduka,

Whenever you have a moment, could you please review my PR? I’d really appreciate it.

[Mamaduka]

Thanks for working on this, @im3dabasia!

Do you mind adding before/after screenshots? You can find examples here - #64586.

[afercia]

There's now a smaller PR #69334 for the InputControl. My fault, I missed this PR and the associated issue #64666.
I'd suggest to merge #69334 first, if no objections. The use this existing PR for a small improvement:

• Move the CSS-in-JS placeholder CSS rules to a new file in https://github.com/WordPress/gutenberg/blob/5c38661db385abdfbc0093a38fb01849d6793e6b/packages/components/src/utils/style-mixins.js and then reuse it in InputControl and TextareControl.
• Having this styling in a centralized place would help future adjustments and maintenance.
• The opacity for Firefox should be removed, see Update CSS for ::-moz-placeholder #69311
• For the scss part, I'm already adding a mixin in Fix the placeholder attribute and caption placeholder color #69333 and I will take care to reuse it for the text control once this PR is merged.