chore(multiple samples) Updating dependencies for Werkzeug and Flask

[XrossFox]

Description

This PR attempts to update all instances of Werkzeug and Flask, since they are commonly used together. With the extra addition of a few airflow samples that use both frameworks in their constraint.txt. The goal is to fix all dependabot security alerts. Failing pipelines that are not because this dependency update shall be addressed in a different, individual PR per failure.

fixes b/514791011 and b/518892254

failures will be documented in b/514353082

Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• README is updated to include all relevant information
• Tests pass: nox -s py-3.9 (see Test Environment Setup)
• Lint pass: nox -s lint (see Test Environment Setup)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[gemini-code-assist]

Code Review

This pull request updates Flask to version 3.1.3 and Werkzeug to version 3.1.8 across numerous sample applications, generally restricting support to Python 3.9 and above. The review feedback identifies several critical issues: first, upgrading Werkzeug to 3.x while leaving Flask pinned at 2.x in several constraints and test files will cause runtime ImportErrors due to removed APIs. Second, the upgrade will break the websockets sample because Flask-Sockets is incompatible with Flask 2.0+. Finally, the changes introduce inconsistencies in migration samples by dropping support for older Python versions for Flask/Werkzeug while retaining legacy pins for other dependencies, potentially breaking the samples for users in those environments.

[XrossFox]

Code Review

This pull request updates Flask to version 3.1.3 and Werkzeug to version 3.1.8 across numerous sample applications, generally restricting support to Python 3.9 and above. The review feedback identifies several critical issues: first, upgrading Werkzeug to 3.x while leaving Flask pinned at 2.x in several constraints and test files will cause runtime ImportErrors due to removed APIs. Second, the upgrade will break the websockets sample because Flask-Sockets is incompatible with Flask 2.0+. Finally, the changes introduce inconsistencies in migration samples by dropping support for older Python versions for Flask/Werkzeug while retaining legacy pins for other dependencies, potentially breaking the samples for users in those environments.

Adressed pins for earlier version: removed since any version under python 3.10 is no longer supported. websockets sample has to be refactored (documented in b/514353082) since Flask-Sockets is no longer maintained.