Update dependency gulp-sourcemaps to v3

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
gulp-sourcemaps	^2.6.5 -> ^3.0.0

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Release Notes

gulp-sourcemaps/gulp-sourcemaps

### [`v3.0.0`](https://togithub.com/gulp-sourcemaps/gulp-sourcemaps/releases/v3.0.0) [Compare Source](https://togithub.com/gulp-sourcemaps/gulp-sourcemaps/compare/v2.6.5...v3.0.0) Nothing changed to the overall API. However the following security fixes lead to the following: - acorn which caused us to drop support for Node < 6 [#​376](https://togithub.com/gulp-sourcemaps/gulp-sourcemaps/pull/376/files) - css bump [#​380](https://togithub.com/gulp-sourcemaps/gulp-sourcemaps/issues/380)

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