Rename ApplicationSetting.database_apdex_settings to be generic

Why this issue?

As part of #357251 (closed), we are introducing WAL rate as a new database health check indicator. Similar to patroni apdex indicator, this hits prometheus to get the values.

Existing database_apdex_settings attribute in application_settings table stores the value of PromQL endpoint and Patroni Apdex config details. We would like to store the WAL rate configurations in the same attribute as well, also any prometheus based indicators getting added later can be stored here. For that this attribute has to be renamed to be more generic.

New column name: prometheus_alert_db_indicators_settings