Use `SemanticsUpdateBuilder` again, remove all references to temporary `SemanticsUpdateBuilderNew` (#139942)

This PR removes all usages of the temporary `SemanticsUpdateBuilderNew` API in favor of `SemanticsUpdateBuilder`. These two APIs are the same as of now.

This is mainly targeted at https://github.com/flutter/flutter/issues/17988

Steps:
part 1: [engine] add `SemanticsUpdateBuilderNew` https://github.com/flutter/engine/pull/47961
part 2: [flutter] use `SemanticsUpdateBuilderNew` https://github.com/flutter/flutter/pull/138331
part 3: [engine] update `SemanticsUpdateBuilder` to be the same as `SemanticsUpdateBuilderNew` https://github.com/flutter/engine/pull/48882
**part 4: [flutter] use (now updated) `SemanticsUpdateBuilder` again** <-- we are here
part 5: [engine] remove `SemanticsBuilderNew`