Improve the snippet editor placeholder color contrast

[afercia]

Summary

This PR can be summarized in the following changelog entry:

• Improve the snippet editor placeholder color contrast

Relevant technical choices:

• Uses the same gray color we use on MyYoast

Test instructions

• currently, the placeholder text in the description field has a light gray color that becomes even lighter when the field is focused:

This PR changes the color to #646464 in both states (normal and focused).

• test the standalone version: yarn-start and go to localhost:3333
• open the snippet editor
• empty the description field to make the placeholder text appear
• check the placeholder color is always #646464 also when the field is focused
• optionally, yarn-link to the plugin release-7.7 branch and test also in the plugin

Fixes #602
Fixes Yoast/wordpress-seo#9874

[afercia]

Connects to Yoast/wordpress-seo#9874

[afercia]

Trying to wake up Waffle:

Fixes Yoast/wordpress-seo#9874

[boblinthorst]

CR: ok 👍

[boblinthorst]

Acceptance: ok 👍