Feat: Turn Down "google/cloud" Package

[bshaffer]

We would like to turn down the google/cloud parent package in favor of installing individual library packages. Some reasons include that 1) some of our libraries support different versions of PHP, 2) we may end up having diamond dependency issues when we start moving libraries to https://github.com/googleapis/google-cloud-php/discussions/5206, and 3) using individual packages is a better and more efficient developer experience. That being said, this package is still receiving 6k downloads/day according to google/cloud packagist stats, so rather than deactivate the package entirely, we should perform the following tasks:

• [ ] Add note to parent README to not require google/cloud
• [ ] Mark package as abandoned using "abandoned": true in composer.json
• [ ] Remove google/cloud installation instructions from package READMEs
• [ ] Stop generating documentation for the umbrella google/cloud package
• [ ] Ensure new git tags do not create a composer package version
• [ ] Search docs and samples to see if google/cloud is recommended anywhere, and recommend sub-repos instead
• [ ] Deprecate Core/ServiceBuilder and reword deprecation message in ServiceBuilder

[pkruithof]

Something you can already do: mark the package as abandoned so installs will get a notification about this. You can also point to a replacement package, though I'm not sure which one that would be since this is a bundle containing all packages.