fix(internal/serviceconfig): port release_level info for java from generation_config.yaml (#4862)

Ports over release_level info from
google-cloud-java/generation_config.yaml to sdk.yaml. Filled out
"preview" for empty values (default in java's current hermetic build
logic), removed "stable" as it is
[default](https://github.com/googleapis/librarian/blob/cec5d8b73019bdf85d7bd0843a74f84897b641c9/internal/serviceconfig/api.go#L167).
Changes are made by running a temporarily script (included in commit
[957dceb](957dceb)
for record sake).
This is a fix after #4835 moved the source of truth for release levels
to sdk.yaml.

Fix #4859

Author: zhumin8