single_line_to_string needs update

With ScalaPB 0.7, singleLineToString has been renamed to singleLineToProtoString. Just one line of code needs updating to be compatible with the latest ScalaPB:
https://github.com/bazelbuild/rules_scala/blob/958864711b3fe00045ac57b2ba86f709eeb8262d/scala_proto/scala_proto.bzl#L484

As it stands now, I get this error when building a scalapb_proto_library:
```
--scala_out: Unrecognized parameter: 'single_line_to_string'
ERROR: /.../BUILD.bazel:19:1: scala //protos/p3/mgmt:mgmt_scala_proto failed (Exit 1)
```

ScalaPB docs that mention singleLineToString change:
https://scalapb.github.io/sbt-settings.html

Start at scala_proto/scala_proto.bzl around line 484, where the single_line_to_string option is referenced. Build a scalapb_proto_library using the issue’s failing example or equivalent fixture and confirm the unrecognized-parameter error is gone with the ScalaPB 0.7 naming.

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