[8.x] Fix standalone agent K8s manifest link to include patch level (backport #1344)

[mergify]

It was reported in Slack that on the Kubernetes install instructions for standalone agent, customers are being directed to 8.15.2 manifest files before they're fully available.

Essentially this link:

curl -L -O https://raw.githubusercontent.com/elastic/elastic-agent/8.15/deploy/kubernetes/elastic-agent-standalone-kubernetes.yaml

Should be:

curl -L -O https://raw.githubusercontent.com/elastic/elastic-agent/8.15.1deploy/kubernetes/elastic-agent-standalone-kubernetes.yaml

I've fixed this by changing the {manifest} variable to include {bare-version} (which is currently set to 8.15.1) rather than {branch} (which is currently set to 8.15).

The screen capture below shows the manifest link as 8.16.0 because this change is going into the main branch. In the public docs, this would be the current patch version, so as I write this that version is currently set to 8.15.1.

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This is an automatic backport of pull request #1344 done by [Mergify](https://mergify.com).

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