Remove Zoekt basic auth from main GitLab Helm chart

What does this MR do and why?

Removes Zoekt basic authentication from the main GitLab Helm chart as it's no longer needed. The gitlab-zoekt subchart has switched to using JWT authentication instead of basic auth.

Related to: gitlab-org/gitlab#572228 (closed)

Changes

This MR performs a complete removal of all Zoekt basic auth references:

Templates

• Removed gitlab.zoekt.gateway.basicAuth.secretName template from templates/_zoekt.tpl
• Deleted charts/gitlab/templates/_zoekt.tpl entirely (only contained basic auth secret mounting logic)
• Removed automatic generation of Zoekt basic auth credentials from templates/shared-secrets/_generate_secrets.sh.tpl
• Removed gitlab.appConfig.zoekt template definition from charts/gitlab/templates/_gitlab.yaml.tpl
• Removed include "gitlab.appConfig.zoekt" statements from:
  • charts/gitlab/charts/webservice/templates/configmap.yml
  • charts/gitlab/charts/sidekiq/templates/configmap.yaml
  • charts/gitlab/charts/toolbox/templates/configmap.yaml

Configuration

• Removed global.zoekt.gateway configuration from values.yaml
• Removed gitlab-zoekt.gateway configuration from values.yaml

Deployments

• Removed all references to gitlab.zoekt.mountSecrets from:
  • charts/gitlab/charts/webservice/templates/deployment.yaml
  • charts/gitlab/charts/sidekiq/templates/deployment.yaml
  • charts/gitlab/charts/toolbox/templates/deployment.yaml

Documentation

• Removed Zoekt basic auth password section from doc/installation/secrets.md (English)
• Removed Zoekt basic auth password section from doc-locale/ja-jp/installation/secrets.md (Japanese)

How to set up and validate locally

1. Install the chart with the changes
2. Verify that no Zoekt basic auth secrets are created
3. Verify that the gitlab-zoekt subchart uses JWT authentication (if installed)