Fix 6c -> carbon ext (#536)

Chandler reminded me of this, the `.carbon` extension is noted here:
https://github.com/carbon-language/carbon-lang/tree/trunk/docs/design/code_and_name_organization#overview

.pre-commit-config.yaml

@@ -40,7 +40,7 @@ repos:
             Exceptions. See /LICENSE for license information.
             SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
           - --custom_format
-          - '\.6c$'
+          - '\.carbon$'
           - ''
           - '// '
           - ''

executable_semantics/BUILD

@@ -94,7 +94,7 @@ EXAMPLES = [
 
 [genrule(
     name = "%s_out" % e,
-    srcs = ["testdata/%s.6c" % e],
+    srcs = ["testdata/%s.carbon" % e],
     outs = ["testdata/%s.out" % e],
     # Suppress command errors.
     cmd = "$(location executable_semantics) $< > $@ 2>&1 || echo EXIT CODE: $$? >> $@",