fix(helm): fix storage_config when use_thanos_objstore is set to true

[yann-soubeyrand]

What this PR does / why we need it:

When setting loki.storage.use_thanos_objstore value to true, storage_config inside the configuration file should contain use_thanos_objstore: true. This PR does this.

Checklist

• [X] Reviewed the CONTRIBUTING.md guide (required)
• [ ] Documentation added
• [ ] Tests updated
• [X] Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• [ ] Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• [ ] If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR