Bump slimmer from 15.3.0 to 15.4.0
Bump slimmer from 15.3.0 to 15.4.0

Bumps [slimmer](https://github.com/alphagov/slimmer) from 15.3.0 to 15.4.0.
- [Release notes](https://github.com/alphagov/slimmer/releases)
- [Changelog](https://github.com/alphagov/slimmer/blob/master/CHANGELOG.md)
- [Commits](https://github.com/alphagov/slimmer/compare/v15.3.0...v15.4.0)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
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