feat(Color): Update purple 600 hex

What does this MR do?

In this Figma file @pedroms and @jeldergl explored updating the hex value of $purple-600 to better fit with the step patterns of other hues. The change is minor and doesn’t create any visual regression.

Before	After
#6b51b8	#694cc0

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• [-] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [-] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [-] If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• [-] Added the ~"component:*" label(s) if applicable.

Relates to: gitlab-org/gitlab-services/design.gitlab.com!2076 (merged)