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

---

Release Notes

gulp-sourcemaps/gulp-sourcemaps

### [`v3.0.0`](https://togithub.com/gulp-sourcemaps/gulp-sourcemaps/releases/tag/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 [https://github.com/gulp-sourcemaps/gulp-sourcemaps/pull/376](https://togithub.com/gulp-sourcemaps/gulp-sourcemaps/pull/376)/files - css bump [#​380](https://togithub.com/gulp-sourcemaps/gulp-sourcemaps/issues/380)

---

Configuration

📅 Schedule: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

🚦 Automerge: Disabled by config. Please merge this manually once you are satisfied.

♻ Rebasing: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 Ignore: Close this PR and you won't be reminded about this update again.

---

• [ ] If you want to rebase/retry this PR, click this checkbox.

---

This PR has been generated by Mend Renovate. View repository job log here.