[datadog] Change check-cluster-name regex

[triviajon]

What this PR does / why we need it:

This PR edits the regular expression used to validate cluster names to allow cluster names which start with a digit or contain underscores.

Context:

On EKS, clusters can be created with flexible naming conventions that include characters typically restricted (e.g., 0-my_cluster). When the agent is deployed without explicitly setting a clusterName, the agent successfully auto-detects the name via the EC2 Instance Metadata Service (IMDS).

However, explicitly setting datadog.clusterName: 0-my_cluster currently triggers validation failures at both the Helm chart and Agent levels.

Related to DataDog/datadog-agent#47189 and https://datadoghq.atlassian.net/browse/CONTINT-5127

Note to reviewers

This will not be merged until the release of v7.78

Checklist

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

• All commits are signed (see: signing commits)
• Chart Version semver bump label has been added (use <chartName>/minor-version, <chartName>/patch-version, or <chartName>/no-version-bump)
• For datadog or datadog-operator chart or value changes, update the test baselines (run: make update-test-baselines)

GitHub CI takes care of the below, but are still required:

• Documentation has been updated with helm-docs (run: .github/helm-docs.sh)
• CHANGELOG.md has been updated
• Variables are documented in the README.md