Fix release tag on Unity SDK installation document page (#2622)

[oniku-2929]

What type of PR is this?

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

/kind breaking
/kind bug
/kind cleanup
/kind feature
/kind hotfix

/kind documentation

What this PR does / Why we need it:
To fix invalid tag information on Unity SDK installation document page.
Also, it automatically increments the release tag based on the release version.

Which issue(s) this PR fixes:

Closes #2622

Special notes for your reviewer:
I checked on Unity 2021.3.16f1 Personal.

[agones-bot]

Build Succeeded 👏

Build Id: e7a5fd03-054a-44b8-b1ef-b45f2a38a4b8

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

• image: us-docker.pkg.dev/agones-images/ci/agones-controller:1.31.0-80c92ad-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.31.0-80c92ad-linux-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.31.0-80c92ad-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.31.0-80c92ad-amd64
• Linux C++ SDK (build): agonessdk-1.31.0-80c92ad-amd64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.31.0-80c92ad-amd64.zip

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

• https://80c92ad-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/3071/head:pr_3071 && git checkout pr_3071
• helm install agones ./install/helm/agones --namespace agones-system --agones.image.release=us-docker.pkg.dev/agones-images/ci --set agones.image.tag=1.31.0-80c92ad-amd64

[markmandel]

That's an excellent catch!

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: markmandel, oniku-2929, 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: 257d4013-ea41-460b-a8e0-44829cdcccea

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

• image: us-docker.pkg.dev/agones-images/ci/agones-controller:1.31.0-cac6ddc-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.31.0-cac6ddc-linux-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.31.0-cac6ddc-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.31.0-cac6ddc-amd64
• Linux C++ SDK (build): agonessdk-1.31.0-cac6ddc-amd64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.31.0-cac6ddc-amd64.zip

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

• https://cac6ddc-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/3071/head:pr_3071 && git checkout pr_3071
• helm install agones ./install/helm/agones --namespace agones-system --agones.image.release=us-docker.pkg.dev/agones-images/ci --set agones.image.tag=1.31.0-cac6ddc-amd64