docs: refresh INSTALLING.md stable version examples

[auyua9]

Summary\n- update the quick-start PyPI install example in from Airflow 3.0.0 to the current stable 3.1.8 release\n- update the matching constraints URL in the plain install example\n- update the matching constraints URL in the extras example\n\n## Why\nThe root quick-start examples still point readers at and , while the current stable install examples in the repo have already moved forward to 3.1.8. This keeps the quick-start copy-paste path aligned with the current stable release surface without changing the surrounding installation guidance.\n\n## Testing\n- git diff --check

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contributors' Guide (https://github.com/apache/airflow/blob/main/contributing-docs/README.rst)
Here are some useful points:

• Pay attention to the quality of your code (ruff, mypy and type annotations). Our prek-hooks will help you with that.
• In case of a new feature add useful documentation (in docstrings or in docs/ directory). Adding a new operator? Check this short guide Consider adding an example DAG that shows how users should use it.
• Consider using Breeze environment for testing locally, it's a heavy docker but it ships with a working Airflow and a lot of integrations.
• Be patient and persistent. It might take some time to get a review or get the final approval from Committers.
• Please follow ASF Code of Conduct for all communication including (but not limited to) comments on Pull Requests, Mailing list and Slack.
• Be sure to read the Airflow Coding style.
• Always keep your Pull Requests rebased, otherwise your build might fail due to changes not related to your commits.
  Apache Airflow is a community-driven project and together we are making it better 🚀.
  In case of doubts contact the developers at:
  Mailing List: dev@airflow.apache.org
  Slack: https://s.apache.org/airflow-slack