misc,tests: Remove support for GCC 6

Supporting GCC 7 onwards will allow us to move to the C++17 standard.

Change-Id: I805945230d77dacdc6b0a863153b28276025ecdc
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/47460
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
diff --git a/tests/compiler-tests.sh b/tests/compiler-tests.sh
index 11b84a0..bf1d281 100755
--- a/tests/compiler-tests.sh
+++ b/tests/compiler-tests.sh
@@ -14,7 +14,6 @@
         "gcc-version-9"
         "gcc-version-8"
         "gcc-version-7"
-        "gcc-version-6"
         "clang-version-11"
         "clang-version-10"
         "clang-version-9"