Remove upstream workflows that don't apply to the GeneXus fork

These workflows (CI Build & Package, Release Build & Package, Push to
NuGet, PR validation) were inherited from upstream/main when 6.0-GX was
seeded. They reference secrets (NUGET_API_KEY) and repository variables
(CI_VERSION_MAJORMINOR, RELEASE_VERSION_MAJORMINOR) that the
genexuslabs fork does not define, so any time they fire they fail. The
GeneXus-owned 'Build' workflow (build.yml) covers all the cases for this
fork (push to **-GX, release published, workflow_dispatch).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: claudiamurialdo