opensearch: Replace templates with Helm dependency

[lindhe]

Description

This change also includes:

• Rewrite of logic for INVENIO_SEARCH_HOSTS.

• Remove secondary INVENIO_SEARCH_HOSTS:

  This was set twice: once via the envFrom: invenio-config and once conditionally via a secret. This change removes the second case.

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• [x] I'm aware of the code of conduct.
• [x] I've created logical separate commits and followed the commit message format.
• [ ] I've added relevant test cases.
• [ ] I've added relevant documentation.
• [ ] I've marked translation strings (for relevant code).
• [ ] I've followed the CSS/JS and React guidelines (for relevant code).
• [ ] I've followed the web accessibility guidelines (for relevant code).
• [ ] I've followed the user interface guidelines (for relevant code).
• [ ] I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions).
• [ ] I've NOT included third-party code (copy/pasted source code or new dependencies).

Third-party code

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