Input placeholder color contrast does not meet WCAG 2.0 AA requirements

[kgibb]

Detailed description

What version of the Carbon Design System are you using?

carbon-components@10.14.0

What did you expect to happen?

Placeholder contrast ratio should be at least 4.5:1.

What WCAG 2.1 checkpoint does the issue violate?

1.4.3 Contrast Minimum Level AA

Placeholder Contrast Ratios

Theme	Light	Background	Foreground	Ratio
white	✔️	#ffffff	#a8a8a8	2.37:1
white	❌	#f4f4f4	#a8a8a8	2.16:1
g10	✔️	#f4f4f4	#a8a8a8	2.16:1
g10	❌	#ffffff	#a8a8a8	2.37:1
g90	✔️	#525252	#6f6f6f	1.55:1
g90	❌	#393939	#6f6f6f	2.29:1
g100	✔️	#393939	#6f6f6f	2.29:1
g100	❌	#262626	#6f6f6f	3.01:1

Related: #5022

[dakahn]

@carbon-design-system/design didn't we address the contrast disparities on various backgrounds with placeholders last year? I thought I remembered us updating those tokens.

[laurenmrice]

The placeholder text color contrast was addressed in this PR.

If the "placeholder" text is actually displaying the format of the input or a prompt then it needs to pass color contrast (ie the date picker component, which is text-05). If however it is non-informational text (ie an example) then it can remain the low contrast placeholder color text-03. More information on this here: #4799 (review)

[dakahn]

Sounds good. closing as resolved -- happy to reopen if that's not the case.