C++ extensions do not work with the old g++ system compiler on OL7 and Ubuntu 16.04

* The error is:
  /test/truffleruby-standalone/lib/cext/include/ruby/backward/cxxanyargs.hpp:98:69: error: 'std::nullptr_t' has not been declared
  rb_define_virtual_variable(const char *q, rb_gvar_getter_t *w, std::nullptr_t e)
* HAVE_NULLPTR is defined to 1 in all platform config files,
  because of using newer CC/CXX in PATH when running the CRuby ./configure script.
* On OL7 it should be possible to install devtoolset to address this, like documented in
  https://github.com/oracle/graal/blob/master/docs/getting-started/graalvm-enterprise/oci/installation-compute-instance-with-OL.md#oracle-linux-7
  However it seems messy and OL7 is EOL soon anyway.

Author: eregon

README.md

@@ -87,7 +87,7 @@ Runtime configurations are further detailed in [Deploying TruffleRuby](doc/user/
 TruffleRuby is actively tested on the following systems:
 
 * Oracle Linux 7, 8, 9
-* Ubuntu 16.04, 18.04, 20.04, 22.04 (all LTS)
+* Ubuntu 18.04, 20.04, 22.04 (all LTS)
 * Fedora 37, 38
 * Debian 10, 11, 12
 * macOS 11 (Big Sur)

tool/docker-configs.yaml

@@ -25,12 +25,13 @@ deb: &deb
     - RUN locale-gen
     - ENV LANG=en_US.UTF-8
 
-ol7:
-  base: oraclelinux:7-slim
-  # --enablerepo needed for libyaml-devel
-  install: RUN yum install --enablerepo=ol7_optional_latest -y
-  locale:
-  <<: *rpm
+# Too old g++
+#ol7:
+#  base: oraclelinux:7-slim
+#  # --enablerepo needed for libyaml-devel
+#  install: RUN yum install --enablerepo=ol7_optional_latest -y
+#  locale:
+#  <<: *rpm
 
 ol8:
   base: oraclelinux:8-slim
@@ -58,11 +59,6 @@ fedora38:
   locale: glibc-langpack-en
   <<: *rpm
 
-ubuntu1604:
-  base: ubuntu:16.04
-  install: RUN apt-get update && apt-get install -y
-  <<: *deb
-
 ubuntu1804:
   base: ubuntu:18.04
   install: RUN apt-get update && apt-get install -y