It seems the single-quotes were copied from logback.groovy, but they are not necessary.
Before this change, the single-quotes were printed to the console, which looked weird:
'2022-01-26 14:40:38.140 WARN --- [ restartedMain] org.grails.config.NavigableMap : Accessing config key '[plugins]' through dot notation is deprecated, and it will be removed in a future release. Use 'config.getProperty(key, targetClass)' instead.
''2022-01-26 14:40:43.486 WARN --- [ restartedMain] org.grails.config.NavigableMap : Accessing config key '[plugins, springsecurity]' through dot notation is deprecated, and it will be removed in a future release. Use 'config.getProperty(key, targetClass)' instead.
'
It seems the single-quotes were copied from logback.groovy, but they are not necessary.
Before this change, the single-quotes were printed to the console, which looked weird: