[Enhancement][OpenSearch Dashboards] Do not hardcode imageTag in values.yaml

[k3a]

Is your feature request related to a problem? Please describe.
imageTag should not be hard-codded to specific version (like 1.0.0).

Describe the solution you'd like
By default, the chart should use the appVersion tag from Chart.yaml just like in openserach Heml chart.
Helm config variable imageTag can still be used for manual overrides if someone needs to use a specific version.

Additional context
Users who just upgraded the chart version in their deployments have not been receiving Opensearch Dashboard updates and were stuck with 1.0.0 unless they manually specified imageTag: "" or imageTag: 1.2.0 in the Helm deployment config.