CI: PR-5.x workflow fails for all PRs - references deleted geometry_module branch

Problem

.github/workflows/PR-5.x.yaml references its reusable workflows pinned to a geometry_module branch of opencv/ci-gha-workflow:

uses: opencv/ci-gha-workflow/.github/workflows/OCV-PR-Linux.yaml@geometry_module

All 8 job definitions in the file use that ref.

That branch no longer exists:

$ gh api repos/opencv/ci-gha-workflow/branches/geometry_module
{"message":"Branch not found","status":"404"}

Because none of the uses: references resolve, GitHub never schedules a job and the run fails immediately with "No jobs were run" / "This run likely failed because of a workflow file issue."

Impact

Every PR-5.x.yaml run has failed since at least 2026-07-23, on unrelated branches including 5.x itself and maintainer branches:

PRs are being merged through it regardless (e.g. disable_engine_classic merged as #4166 with this check red), but every PR targeting 5.x currently shows a red X that carries no information about the change itself, and contributors have no CI signal.

Suggested fix

opencv/opencv's own .github/workflows/PR-5.x.yaml already points the same set of reusable workflows at @main:

uses: opencv/ci-gha-workflow/.github/workflows/OCV-PR-Linux.yaml@main

Repointing the refs in this repository from @geometry_module to @main should restore the checks. It looks like the file was pinned to a temporary feature branch during the geometry module migration and not updated after that branch was removed.