ci: remove redundant single-entry matrix in test-podman-next workflow

[Ankitsinghsisodya]

Problem

.github/workflows/test-podman-next.yaml defines a strategy matrix with only one entry:

strategy:
  matrix:
    os: ["ubuntu-latest"]
runs-on: ${{ matrix.os }}

A matrix with a single element adds unnecessary complexity without any benefit. It makes the workflow harder to read and understand.

Fix

Remove the strategy.matrix block and use runs-on: ubuntu-latest directly.

[Ankitsinghsisodya]

/assign