Undeprecate schemaFile

[martinbonnin]

I changed my mind. There is a global expectation that there is a "main" schema file. Furthermore, deprecating this also had cascading impacts on introspection {} for which we don't have a good answer right now.

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[martinbonnin]

Drawback: using schemaFile.set() ignores any extra.graphqls file.

[martinbonnin]

using schemaFile.set() ignores any extra.graphqls file

I just checked 3.x and it's already the case so let's err on the conservative side and keep things as is for the time being.

The current logic is:

1. files are auto-detected from src/main/graphql
2. srcDir overrides that you to look into other directories
3. schemaFiles overrides srcDir for schema files
4. Service.schemaFile overrides schemaFiles
5. introspection {} uses the result of the above
   a. unless there are multiple schema files found
   a. or Instrospection.schemaFile is set itself

Long term I think we'd want only 2 modes:

1. simple: just use what's in src/main|commonMain/graphql
2. advanced:
   a. specify .graphql files with graphqlFiles(FileCollection)
   a. specify .graphqls files with schemaFiles(FileCollection)
   a. introspection is dumb and requires outputFile to be set explicitely.