Documentation: Correct blocked SMTP ports for GKE

What does this MR do?

Correct the note on blocked ports at GKE / GCP. They no longer block 465 and 587 outgoing. See their documentation: https://cloud.google.com/compute/docs/tutorials/sending-mail/#choosing_an_email_service_to_use

Tested on GKE using adhoc container:

root@tmp-579f46f469-pc8fg:/# curl -m 5 portquiz.net:25
curl: (28) Connection timed out after 5001 milliseconds
root@tmp-579f46f469-pc8fg:/# curl -m 5 portquiz.net:465
Port 465 test successful!
Your IP: 35.204.xxx.xxx
root@tmp-579f46f469-pc8fg:/# curl -m 5 portquiz.net:587
Port 587 test successful!
Your IP: 35.204.xxx.xxx

Related issues

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards