[java-cloud-bom] Promote to Beta

Package name: **google-cloud-bom**
Current release: **alpha**
Proposed release: **beta**

## Instructions

Check the lists below, adding tests / documentation as required. Once all the "required" boxes are ticked, please create a release and close this issue.

## Required

- [ ] Server API is beta or GA
- [ ] Service API is public
- [ ] Client surface is mostly stable (no known issues that could significantly change the surface)
- [ ] All manual types and methods have comment documentation
- [ ] Package name is idiomatic for the platform
- [ ] At least one integration/smoke test is defined and passing
- [ ] Central GitHub README lists and points to the per-API README
- [ ] Per-API README links to product page on cloud.google.com
- [ ] Manual code has been reviewed for API stability by repo owner

## Optional

- [ ] Most common / important scenarios have descriptive samples
- [ ] Public manual methods have at least one usage sample each (excluding overloads)
- [ ] Per-API README includes a full description of the API
- [ ] Per-API README contains at least one “getting started” sample using the most common API scenario
- [ ] Manual code has been reviewed by API producer
- [ ] Manual code has been reviewed by a DPE responsible for samples
- [ ] 'Client LIbraries' page is added to the product documentation in 'APIs & Reference' section of the product's documentation on Cloud Site