feat!: remove Python 2.7 from tested versions

[glasnt]

With #13596, we both added Python 3.14, but also removed python 2.7 from the base testing image.

The way testing is currently setup, while most tests (apart from App Engine Standard) are configured to skip Python 2.7, nox only handles this skip within the versioned environment.

For example, previously these checks would show:

cp /workspace/noxfile-template.py noxfile.py
cd path/to/sample
nox -s py-2.7
nox > Running session py-2.7
nox > Creating virtual environment (virtualenv) using python2.7 in .nox/py-2-7
nox > Session py-2.7 skipped: SKIPPED: 2.7 tests are disabled for this sample..

Since the merge of 13596, these now appear as:

cd path/to/sample
nox -s py-2.7
nox > Running session py-2.7
nox > Session py-2.7 failed: Python interpreter 2.7 not found.

With the deprecation of Python 2.7 for App Engine Standard first-generation just over 6 weeks away, we should be able to remove this version from our Nox and Kokoro configurations, thus preventing this failing error.

Additionally, since the copy of noxfile-template in App Engine Standard was primarily there because of Python 2.7, this forked version has been deleted.

⚠️ As a temporary workaround, the "Kokoro CI - Python 2.7 (App Engine Standard Only)" test has been removed as being "Required" to allow a PR to be merged.

• Confirm cl/846023750 resolved
• Please merge this PR for me once it is approved

[gemini-code-assist]

Summary of Changes

Hello @glasnt, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request finalizes the removal of Python 2.7 support from the project's testing infrastructure. Following a previous update that removed Python 2.7 from the base testing image, this change updates the Nox configuration to no longer include Python 2.7 in the list of tested versions, preventing test failures due to missing interpreters. Additionally, a Python 2.7-specific Noxfile template for App Engine Standard has been removed.

Highlights

• Python 2.7 Removal: Python 2.7 has been explicitly removed from the ALL_VERSIONS list in noxfile-template.py, aligning with its prior removal from the base testing image and upcoming deprecation for App Engine Standard.
• App Engine Standard Noxfile Deletion: The appengine/standard/noxfile-template.py file, which was primarily maintained for Python 2.7 testing within App Engine Standard, has been completely removed.

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[gemini-code-assist]

Code Review

This pull request correctly removes Python 2.7 from the tested versions to align with its deprecation and removal from the testing environment. The changes, which include updating ALL_VERSIONS in noxfile-template.py and deleting the specialized appengine/standard/noxfile-template.py, are logical and directly address the issue of failing builds. The implementation is sound and I have no concerns.