[synthetics] Bump to 0.16.4 with PL 1.49.0

[ruizb]

What this PR does / why we need it:

Bump the Synthetics private location chart to 0.16.4 for the new PL 1.49.0 release.

Checklist

• [x] Chart Version bumped
• [x] Documentation has been updated with helm-docs (run: .github/helm-docs.sh)
• [x] CHANGELOG.md has been updated
• [x] Variables are documented in the README.md
• [ ] For Datadog Operator chart or value changes update the test baselines (run: make update-test-baselines)