Kubernetes secrets should not be stored as environment variables.

[AleksandarSavchev]

What would you like to be added:
Currently gardener-dashboard stores multiple secrets as environment variables: SESSION_SECRET, OIDC_CLIENT_ID, OIDC_CLIENT_SECRET, GITHUB_AUTHENTICATION_APP_ID, GITHUB_AUTHENTICATION_CLIENT_ID, GITHUB_AUTHENTICATION_CLIENT_SECRET, GITHUB_AUTHENTICATION_INSTALLATION_ID, GITHUB_AUTHENTICATION_PRIVATE_KEY, GITHUB_WEBHOOK_SECRET.

I would like these secrets to not be stored as environment variables. A solution can be for the secrets to be mounted as config/secret files and read from there.

Why is this needed:
Gardener aims to comply with DISA K8s STIGs. This issue is in sync with rule 242415.