Gulp: use 'gulp.series' in the single process branch of createMainBatch

[San4es]

Fixes Gulp failures such as:

[08:39:37] Error in plugin 'webpack-stream'
Message:
    ./artifacts/transpiled/ui/scheduler/workspaces/ui.scheduler.work_space.js
Module parse failed: Unexpected token (202:11)
You may need an appropriate loader to handle this file type.
|   _getRightCell: function _getRightCell(isMultiSelection) {
|     if (!isDefined(this._$focusedCell)) {
|       retur
 @ ./artifacts/transpiled/ui/scheduler/workspaces/ui.scheduler.agenda.js 15:25-61 

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Additional details:

• Concurrent Compilation Error shama/webpack-stream#201 (comment)
• gulp.parallel doesn't make much sense for these resource-intensive tasks in the single process mode