chore: update CODEOWNERS

[happynancee]

What does this PR do?

Add APM Serverless team as equal co-codeowners to the repo

Motivation

https://datadoghq.atlassian.net/browse/APMSVLS-28

Update ownership according to the Serverless/APM Ownership doc

Testing Guidelines

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)