[8.x](backport #6293) chore: update version and appVersion in the elastic-agent helm chart

[mergify]

What does this PR do?

This PR extends the logic of mage helm:updateAgentVersion to include updating the version (chart version) and appVersion keys in the Chart.yaml of the Elastic-Agent Helm chart. Note that since the chart is not yet GA for the chart version we always append -SNAPSHOT to mark it as devel. This will have to change when we reach the version that the Helm chart is GA.

Why is it important?

It is important because it sets correctly the chart and the app version in the Chart.yaml of the Elastic-Agent Helm chart and this is required before being able to release the Helm chart.

Checklist

• 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

N/A

How to test this PR locally

change either or both version and appVersion in deploy/helm/elastic-agent/Chart.yaml and then run mage helm:updateAgentVersion. The two fields will get updated to reflect the correct values based on the branch you are.

Related issues

• Closes Mark the Elastic Agent Helm Chart as beta #5485
• Relates [Discuss] Elastic Agent Helm Chart release process #5486

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This is an automatic backport of pull request #6293 done by [Mergify](https://mergify.com).

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
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