doc: reference doc not pointing to latest

[zhumin8]

documentation page with version specified works fine. e.g., https://googlecloudplatform.github.io/spring-cloud-gcp/6.0.1/reference/html/index.html

But if no version specified, use this link: https://googlecloudplatform.github.io/spring-cloud-gcp/reference/html/index.html
it links to 4.8.5-SNAPSHOT, see screenshot

This link is used in https://cloud.google.com/java/docs/spring.

[zhumin8]

From a quick search, last change to ghpages.sh was to disable ref doc publishing for snapshot versions here: #2370

It's possible that we used to only publish snapshot versions via this link without versions.

If this theory is correct, we have a few options:

• update ref publishing script, to also publish latest version to this link without versions
• to remove contents for this link , and link versioned pages to the guide. (less desired, this would require updates per release)