use Elastic demo images for Helm deployment

[rogercoll]

Changes

Changes default Helm image value to use Elastic's images.

Merge Requirements

For new features contributions please make sure you have completed the following
essential items:

• CHANGELOG.md updated to document new feature additions
• Appropriate documentation updates in the docs
• Appropriate Helm chart updates in the helm-charts

Maintainers will not merge until the above have been completed. If you're unsure
which docs need to be changed ping the
@open-telemetry/demo-approvers.

[ChrsMark]

Do we have any kind of documentation of how this work and how it could be overridden?

[rogercoll]

Do we have any kind of documentation of how this work and how it could be overridden?

This value is used to replace the image source used in the Helm chart: https://github.com/open-telemetry/opentelemetry-helm-charts/tree/main/charts/opentelemetry-demo#default-parameters-applied-to-all-demo-components

Currently, we are just documenting the instructions to launch our overwritten values: https://github.com/elastic/opentelemetry-demo?tab=readme-ov-file#start-the-demo

I am working on another PR to disable opensearch and modify the collector's configuration in the same file, if it works for you, I will add a contributing/changes section in the README to denote all overwritten values in that PR.