Remove the warning about the unity SDK not being feature complete.

[roberthbailey]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind breaking
/kind bug

/kind cleanup

/kind documentation
/kind feature
/kind hotfix

What this PR does / Why we need it:
Removes an old warning that is out of date.

Which issue(s) this PR fixes:

Closes #1809

Special notes for your reviewer:
I didn't add the release guards to the website docs since this is fixing an issue that should have been published many releases ago.

[roberthbailey]

/assign @markmandel

[markmandel]

Gah! I was just doing this too 🤣

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: markmandel, roberthbailey

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [markmandel,roberthbailey]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[agones-bot]

Build Succeeded 👏

Build Id: da36ba7c-150f-4aff-90a4-a133102f2310

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:1.9.0-9509191
• image: gcr.io/agones-images/agones-sdk:1.9.0-9509191
• image: gcr.io/agones-images/agones-ping:1.9.0-9509191
• Linux C++ SDK (build): agonessdk-1.9.0-9509191-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.9.0-9509191.zip

A preview of the website (the last 30 builds are retained):

• https://9509191-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1817/head:pr_1817 && git checkout pr_1817
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.9.0-9509191