[9.0](backport #4402) Use chi/middleware.Throttle instead of custom conn limiter

[mergify]

What is the problem this PR solves?

fleet-server silently closes connections instead of returning a 429 when the connection limit is reached.

How does this PR solve the problem?

Use chi/middleware.Throttle to track in-flight requests and return a 429 response once the limit is reached.

This is different behaviour from our current implementation where the check occurs when the TCP connection is established, before the TLS handshake occurs.

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

• Closes Fleet-Server abruptly closes connection when limit is reached instead of returning TooManyRequests (429) #4200

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  This is an automatic backport of pull request Use chi/middleware.Throttle instead of custom conn limiter #4402 done by Mergify.

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube