Add note about cross site cookies browser limitation to Jira App page

What does this MR do?

It adds a note to the Jira App saying:

Adding a namespace currently works only in browsers that allow cross site cookies. Please make sure to use Firefox or Google Chrome or enable cross-site cookies in your browser when adding a namespace.

Adding the namespace only works in browsers that allow cross site cookies because the page is loaded via iframe within Jira.

Screenshots

Related issues

#263509 (closed)

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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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