Ensure JSON is used for Sentry requests

What does this MR do?

This fixes the client calls to Sentry to make sure it works for both Sentry.com and self-hosted Sentry instances.

Before this fix the Sentry update would work on Sentry.com, but did not seem to work for self-hosted instances such as sentry.gitlab.net.

Upon further investigation we were receiving 415 Unsupported Media Type responses.

Changing the request to send Content-Type: application/json, and for PUT (and POST in the future) ensuring that the body is a JSON hash seems to fix the issue.

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

Closes #196874 (closed)