[8.18](backport #7011) fix: set gateway k8s replicas spec

[mergify]

What does this PR do?

If the spec.replicas field is not set for the gateway OpenTelemetry Collector, the horizontal Pod autoscaler fails with the following error:

Conditions:
  Type         Status  Reason          Message
  ----         ------  ------          -------
  AbleToScale  False   FailedGetScale  the HPA controller was unable to get the target's current scale: Internal error occurred: the spec replicas field ".spec.replicas" does not exist
Events:
  Type     Reason          Age                     From                       Message
  ----     ------          ----                    ----                       -------
  Warning  FailedGetScale  107s (x779 over 3h14m)  horizontal-pod-autoscaler  Internal error occurred: the spec replicas field ".spec.replicas" does not exist

The configuration will still deploy a gateway collector pod but not the minimum required neither won't be able to scale the pods accordingly.

Why is it important?

Fixes the deployment of the k8s Otel operator.

Checklist

• I have read and understood the pull request guidelines of this project.
• My code follows the style guidelines of this project
• 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
• I have added an integration test or an E2E test

Disruptive User Impact

How to test this PR locally

Related issues

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

---

This is an automatic backport of pull request #7011 done by [Mergify](https://mergify.com).

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

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[elasticmachine]

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cc @rogercoll