chore: remove firewalls in the quickstart cluster cleanup

[danistrebel]

Description

What's changed, or what was fixed?

• clean up firewalls for the GKE cluster (usually removed automatically but might linger around if the cleanup got interrupted)
• Note on cleanup script in the main README

Issues Fixed

• Fixes #626

Housekeeping

(please check all that apply [x], do not edit the text)

• [x] I have run all the tests locally and they all pass.
• [x] I have followed the relevant style guide for my changes.

Full Repo Validation Required

(please check all that apply [x], do not edit the text)

• [ ] PR requires full pipeline run (Run for changes only by default).

CC: @apigee-devrel-reviewers

[danistrebel]

/gcbrun

[apigee-devrel-bot]

Pipeline Report

Pipeline	Result	Elapsed Wall Time
tools/hybrid-quickstart	pass	2529s
TOTAL PIPELINE	pass	2529s

View details in Cloud Build (permission required)

Commit version: f49853e

[danistrebel]

/gcbrun

[apigee-devrel-bot]

Pipeline Report

Pipeline	Result	Elapsed Wall Time
tools/hybrid-quickstart	pass	2283s
TOTAL PIPELINE	pass	2283s

View details in Cloud Build (permission required)

Commit version: 4432390