Bump govuk_app_config from 9.15.5 to 9.15.6 #4170

Job	Run time
test-ruby-matrix (4, 0)	2m 11s
test-ruby-matrix (4, 1)	3m 45s
test-ruby-matrix (4, 2)	2m 30s
test-ruby-matrix (4, 3)	1m 58s
Dependency Review scan / dependency-review-pr	34s
Dependency Review scan / dependency-review-push	0s
Lint SCSS / Run Stylelint	12s
CodeQL SAST scan / Analyze	1m 20s
Security Analysis / Run Brakeman	29s
Lint JavaScript / Run Standardx	10s
Lint Ruby / Run RuboCop	19s
Test JavaScript / Run Jasmine	29s
Test Ruby	1s
	13m 58s

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