rebootstrap: disable gcc-8 builds

Switch diffoscope jobs to gcc-9. The gcc-8 builds stopped working a
while ago and were only kept for reference.

job-cfg/rebootstrap.yaml.py

@@ -36,8 +36,8 @@ release_architectures = """
 
 architectures += mono_architectures
 
-gcc_versions = ("8", "9")
-diffoscope_gcc_versions = ("8",)
+gcc_versions = ("9",)
+diffoscope_gcc_versions = ("9",)
 
 # repository url doubly maintained in bin/rebootstrap.sh
 print("""