Alter checkin API to remove attributes set by audit/unenroll API

[michel-laterman]

What is the problem this PR solves?

Follow up to #3818, alters the checkin API to remove attributes that the audit/unenroll endpoint sets

How does this PR solve the problem?

Alter the checkin wrapper's flush method to include bulk update calls to remove the attributes for agents that have a non-empty audit_unenrolled_reason.

How to test this PR locally

1. Enroll an agent
2. Use the audit/unenroll API to annotate the agent document.
3. Use the checkin api to signal the agent is healthy

Agent document in Elasticsearch should no longer have the audit_unenrolled_reason or associated attributes

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

Related issues

• Relates [Feature Request] Have Elastic Agent send a final message to its fleet server when making changes elastic-agent#484

[michel-laterman]

buildkite test this

[elastic-sonarqube]

Quality Gate passed

Issues
2 New issues
1 Fixed issue
0 Accepted issues

Measures
0 Security Hotspots
88.6% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube

[michel-laterman]

buildkite run perf-tests

[michel-laterman]

buildkite run perf-tests

[michel-laterman]

I realized that the perf-tests here don't actually use the changed endpoint; i'll work on altering them to do so in other prs