Update Index configuration

[josecelano]

Depends on:

• Configuration overhaul: rename auth::secret_key to auth::user_claim_token_pepper torrust-index#653
• Config overhaul: make some fields mandatory torrust-index#673
• Config overhaul: use an explicit section for metadata torrust-index#672

After finishing the last configuration changes in the Index, those last changes must be applied here.

1. The metadata section was changed:

[metadata]
app = "torrust-index"
purpose = "configuration"
schema_version = "2.0.0"

2. secret_key option was renamed to user_claim_token_pepper.

3. And some options were made mandatory:

[metadata]
schema_version = "2.0.0"

[logging]
threshold = "info"

4. Env var changed

From TORRUST_INDEX_CONFIG_OVERRIDE_AUTH__SECRET_KEY to TORRUST_INDEX_CONFIG_OVERRIDE_AUTH__USER_CLAIM_TOKEN_PEPPER