docs: label legacy Storage Classes in documentation

[frankyn]

Cloud Storage storage classes are listed in library javadocs and they need to be updated to label legacy storage with current guidance on using "STANDARD" for new buckets.

Legacy Storage Classes:

• DURABLE_REDUCED_AVAILABILITY
• MULTI_REGIONAL
• REGIONAL

@dmitry-fa can you pick up this change?

[dmitry-fa]

@frankyn yes, I'll take care of it.