Add `.gitlab-ci.yml` extension to the dotNET-Core.yml CI template file

The following discussion from !59023 (merged) should be addressed:

• @shinya.maeda started a discussion:

  Since this file doesn't have .gitlab-ci.yml extension in the file name, this template would not be discoverable from UI nor include. So simply put, it seems deadcode, probably we should delete.

  Reference: d0582f7e

/cc @marcel.amirault

added citemplates label

mentioned in merge request !64901 (merged)

Created !64901 (merged) to address this.

assigned to @marcel.amirault

changed milestone to %14.1

added devopsverify grouppipeline authoring typemaintenance labels

added typefeature label

added sectionops label

closed

removed typefeature label