feat: Add revisionHistoryLimit to datadog-operator Helm chart

[Invincibear]

What this PR does / why we need it:

Adds a configurable revisionHistoryLimit parameter to the datadog-operator chart,
wired into the Operator Deployment's spec.revisionHistoryLimit. This lets users tune
how many old ReplicaSets Kubernetes retains for rollback (e.g. to reduce clutter in
GitOps/CI environments).

The value defaults to 10, matching the Kubernetes default, so the rendered manifest is
unchanged for anyone who doesn't set it — this is a purely additive, backward-compatible
change.

Special notes for your reviewer:

Checklist

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

• All commits are signed and show as "Verified" on GitHub (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)
• For datadog chart changes, received ✅ from a member of your team

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

[Invincibear]

@gabedos would you be the right person to assign this to?

[Invincibear]

@khewonc would you be the right person to assign this to?