Set default log levels for fresh and existing installs

[Montekkundan]

• Seed default log-level config values for fresh installs:
  • database_log_level: warning
  • request_log_level: warning
  • exception_log_level: warning
  • profiler_log_level: none
• Add a SQL patch to backfill missing, NULL, or blank log-level values on existing installs without overriding explicit values.
• Update runtime log-level fallback behavior so missing values use predictable defaults instead of silently falling back to none.
• Use NullLogger when request logging is explicitly set to none.

Testing instructions

• vendor/bin/phpunit --configuration test/phpunit.xml test/unittests/LogSettingsTest.php

#10401