fix(a11y): correct people-picker placeholder luminosity contrast

[Mnickii]

Closes #3128

PR Type

Bugfix

Description of the changes

PR checklist

• Project builds (yarn build) and changes have been tested in at least two supported browsers (Edge + non-Chromium based browser)
• All public APIs (classes, methods, etc) have been documented following the jsdoc syntax
• Stories have been added and existing stories have been tested
• Added appropriate documentation. Docs PR:

• License header has been added to all new source files (yarn setLicense)
• Contains NO breaking changes

Other information

[microsoft-github-policy-service]

Thank you for creating a Pull Request @Mnickii.

This is a checklist for the PR reviewer(s) to complete before approving and merging this PR:

• I have verified a documentation PR has been linked and is approved (or not applicable)
• I have ran this PR locally and have tested the fix/feature
• I have verified that stories have been added to storybook (or not applicable)
• I have tested existing stories in storybook to verify no regression has occured
• I have tested the solution in at least two browsers (Edge + 1 non-Chromium based browser)

[github-actions]

📖 The updated storybook is available here

[github-actions]

📖 The updated storybook is available here

[Mnickii]

@vagpt @Raisul123 please confirm that this fixes the contrast issue

[github-actions]

📖 The updated storybook is available here

[github-actions]

📖 The updated storybook is available here

[github-actions]

📖 The updated storybook is available here

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
mgt-components.src.components	86%	75%	0	✔
mgt-components.src.components.mgt-contact	68%	83%	0	➖
mgt-components.src.components.mgt-file	62%	100%	0	➖
mgt-components.src.components.mgt-file-list	46%	100%	0	❌
mgt-components.src.components.mgt-file-list.mgt-file-upload	49%	86%	0	❌
mgt-components.src.components.mgt-get	22%	100%	0	❌
mgt-components.src.components.mgt-messages	68%	100%	0	➖
mgt-components.src.components.mgt-organization	47%	100%	0	❌
mgt-components.src.components.mgt-person	82%	100%	0	✔
mgt-components.src.components.mgt-person-card	77%	49%	0	✔
mgt-components.src.components.mgt-picker	80%	100%	0	✔
mgt-components.src.components.mgt-profile	40%	100%	0	❌
mgt-components.src.components.mgt-tasks-base	87%	100%	0	✔
mgt-components.src.components.mgt-theme-toggle	100%	100%	0	✔
mgt-components.src.components.mgt-todo	79%	100%	0	✔
mgt-components.src.components.sub-components.mgt-flyout	78%	53%	0	✔
mgt-components.src.components.sub-components.mgt-spinner	100%	100%	0	✔
mgt-components.src.graph	38%	78%	0	❌
mgt-components.src.styles	92%	80%	0	✔
mgt-components.src.utils	79%	27%	0	✔
mgt-element.dist.es6.components.src.components	72%	73%	0	➖
mgt-element.dist.es6.mock.src.mock	91%	73%	0	✔
mgt-element.dist.es6.providers.src.providers	85%	76%	0	✔
mgt-element.dist.es6.src	91%	80%	0	✔
mgt-element.dist.es6.utils.src.utils	68%	71%	0	➖
mgt-element.src	93%	40%	0	✔
mgt-element.src.components	78%	100%	0	✔
mgt-element.src.mock	81%	56%	0	✔
mgt-element.src.providers	84%	100%	0	✔
mgt-element.src.utils	72%	91%	0	➖
Summary	67% (14551 / 21559)	74% (519 / 699)	0	➖