Improve color contrast of light theme entity var

[ctmayn]

Summary

Adjust the value of entity highlighting so it meets the required color contrast on a diff deletion background.

This PR adjusts it from color-scale-purple-5 to color-scale-purple-6

Before:

After:

Fixes https://github.com/github/accessibility-audits/issues/3367

List of notable changes:

• Update the color value of prettylights.syntax.entity to color-scale-purple-6

What should reviewers focus on?

• Color changes and correct changeset

Steps to test:

Contributor checklist:

• All new and existing CI checks pass
• Tests prove that the feature works and covers both happy and unhappy paths
• Any drop in coverage, breaking changes or regressions have been documented above
• All developer debugging and non-functional logging has been removed
• Related issues have been referenced in the PR description

Reviewer checklist:

• Check that pull request and proposed changes adhere to our contribution guidelines and code of conduct
• Check that tests prove the feature works and covers both happy and unhappy paths
• Check that there aren't other open Pull Requests for the same update/change
• Verify the design tokens changed in this PR are expected using the diffing results in this PR

[changeset-bot]

🦋 Changeset detected

Latest commit: 1261bba

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/primitives	Patch

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Click here if you're a maintainer who wants to add another changeset to this PR

[langermank]

Since we're in the middle of testing new color tokens, you'll need to make this change in two places: the file you already changed, and this one. Let me know when you make that change and I'll review again 😄