Remove misleading gitlab-foss reference in README.md

What does this MR do?

After the single repo migration from Gitlab EE and CE, all FOSS & enterprise development takes place in one repo.

This MR removes the misleading reference and link to the previous read-only Gitlab Community Edition project in the Gitlab Readme file.

• Having a link to the obsolete, read-only gitlab-foss repo right at the top is misleading for users
• The word 'FOSS' misleads people to believe the current repo isn't free and open source software.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• Separation of EE specific content

Performance and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team