Renumber exercises in files C-G.

LambdaLab/test/exercises/C_DefaultMethods.java

LambdaLab/test/exercises/D_SimpleStreams.java

LambdaLab/test/exercises/E_IntermediateStreams.java

LambdaLab/test/exercises/F_AdvancedStreams.java

LambdaLab/test/exercises/G_Challenges.java

LambdaLab/test/solutions/C_DefaultMethods.java

LambdaLab/test/solutions/D_SimpleStreams.java

LambdaLab/test/solutions/E_IntermediateStreams.java

LambdaLab/test/solutions/F_AdvancedStreams.java

LambdaLab/test/solutions/G_Challenges.java