Remove HA warning when existingClusterAgent.join is true

[LQss11]

What this PR does / why we need it:

This PR updates the datadog Helm chart Notes.txt to prevent displaying the high availability warning when existingClusterAgent.join is set to true. This adjustment ensures that users following the mixed-clusters-with-the-datadog-cluster-agent example are not misled by unnecessary warnings.

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes #

Special notes for your reviewer:

The warning message will no longer appear when existingClusterAgent.join is enabled.:

###################################################################################
####   WARNING: Cluster-Agent should be deployed in high availability mode     ####
###################################################################################

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• Chart version updated
• Documentation updated (.github/helm-docs.sh run)
• CHANGELOG.md updated
• Variables documented in README.md
• Test baselines updated (make update-test-baselines run)