Clean up CI: simplify dev versioning and remove dead config

[shchur]

What

Housekeeping pass over CI/build config, removing things that unambiguously serve no purpose.

Versioning

• Replace the VERSION.minor date-stamp machinery in setup.py with a plain x.y.z.dev0 suffix for non-release builds. RELEASE=1 (tagged release) → 0.5.1; otherwise → 0.5.1.dev0. Matches AutoGluon's scheme, minus the nightly-counter logic we don't need (no nightly publish for autogluon.cloud). Deletes the tracked-but-gitignored VERSION.minor file and its .gitignore entry.

Dead install machinery

• Remove the unused install_tabular / install_multimodal block from test_cloud.sh and the install_* / install_latest_* helpers from env_setup.sh. The DLC already ships AutoGluon, so the CI host only needs autogluon.cloud[tests] (which pulls autogluon.common).

Triggers / actions

• Drop the push trigger on the nonexistent dev branch (now master, stable, v* tags).
• Align aws-actions/configure-aws-credentials to @v5 in the composite action (was @v1 — inconsistent with the rest of the repo and on the deprecated Node16 runtime).

Build/test config

• Drop tox and pytest-cov from the tests extra (never invoked).
• Drop pypandoc / packaging from the PyPI release build deps (unused).
• Remove [tool.mypy] and [tool.coverage.*] from pyproject.toml (never run).

Kept intentionally

• The disabled (if: false) test_multimodal_cloud and test_cluster jobs remain as placeholders for future re-enablement, along with test_cluster.sh and the pytest-forked install its --forked run needs.

Verification

• Both workflow YAMLs and pyproject.toml parse.
• update_version produces 0.5.1.dev0 (unset) and 0.5.1 (RELEASE=1).
• setup.py passes ruff format/check; pre-commit hooks pass.