Move Python 2.6 unit tests to Github Actions (#17279)

* Run Python2.6 unit tests on Github Actions
* Skip url tests on Python 2.6 to reduce waiting times
* Skip foreground background tests on Python 2.6 to reduce waiting times
* Removed references to Travis in the documentation
* Deleted install_patchelf.sh (can be installed from repo on CentOS 6)

share/spack/qa/install_patchelf.sh

@@ -1,22 +0,0 @@
-#!/bin/sh
-#
-# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
-# Spack Project Developers. See the top-level COPYRIGHT file for details.
-#
-# SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
-#
-# Description:
-#     Install patchelf for use in buildcache unit tests
-#
-# Usage:
-#     install-patchelf.sh
-#
-set -ex
-if [ "$TRAVIS_OS_NAME" = "linux" ]; then
-    olddir=$PWD
-    cd /tmp
-    wget https://github.com/NixOS/patchelf/archive/0.10.tar.gz
-    tar -xvf 0.10.tar.gz
-    cd patchelf-0.10 && ./bootstrap.sh && ./configure --prefix=/usr && make && sudo make install && cd $olddir
-fi