Update pipeline.tests-staging.yaml

[juliaElastic]

What is the problem this PR solves?

Synthetic monitor max polls variable not taking effect.

// Please do not just reference an issue. Explain WHAT the problem this PR solves here.

How does this PR solve the problem?

Fix quality gate variable, it was renamed, see https://github.com/elastic/serverless-quality-gates/issues/76

// Explain HOW you solved the problem in your code. It is possible that during PR reviews this changes and then this section should be updated.

How to test this PR locally

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

[jlind23]

@juliaElastic once #3709 (review) is merged you should be able to rerun your tests and get a green CI.

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube