Resolve "Clean up Cohorts"

What does this MR do?

Frontend: Cleans up the cohorts table as depicted by the provided screenshots.
Backend: Exclude the current month's activity data.

Further details can be found in the issue: #18668 (closed)

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability 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

Todo

• Rename "Cohort total" to "New users"
• Remove the "Month 0" column.
• Add a row above the Month 1 - Month 11 columns that reads "Returning users".
• Remove the Month 0 column.
• Add a separator between Cohort total and Month 1.
• Make the header taller.
• Add the Returning users label in bold text, aligned with the Month 1 column.