Drop support for CentOS 6

CentOS 6 went out of support on November 30, 2020.  Moreover, building
packages for it requires that we build Ruby in addition to all the
gems, plus a version of Git, meaning we spend significant CI resources
and time on this platform.

Since there's no longer any free security support and we cannot
reasonably be expected to run insecure code in CI, since that's
dangerous to others on the Internet, let's drop support for CentOS 6.

docker/run_dockers.bsh

@@ -58,7 +58,7 @@ done
 
 if [[ ${#IMAGES[@]} == 0 ]]; then
   # If you change this list, change script/upload as well.
-  IMAGES=(centos_6 centos_7 centos_8 debian_8 debian_9 debian_10)
+  IMAGES=(centos_7 centos_8 debian_8 debian_9 debian_10)
 fi
 
 mkdir -p "${PACKAGE_DIR}"

script/packagecloud.rb

@@ -27,10 +27,6 @@
 # https://packagecloud.io/docs#os_distro_version
 $distro_name_map = {
   # RHEL EOL https://access.redhat.com/support/policy/updates/errata
-  "centos/6" => [
-    "el/6", # End of Extended Support June 30, 2024
-    "scientific/6",
-  ],
   "centos/7" => [
     "el/7",
     "scientific/7",

script/upload

@@ -185,7 +185,6 @@ finalize_body_message () {
 
 Up to date packages are available on [PackageCloud](https://packagecloud.io/github/git-lfs) and [Homebrew](http://brew.sh/).
 
-[RPM RHEL 6/CentOS 6](https://packagecloud.io/github/git-lfs/packages/el/6/git-lfs-VERSION-1.el6.x86_64.rpm/download)
 [RPM RHEL 7/CentOS 7](https://packagecloud.io/github/git-lfs/packages/el/7/git-lfs-VERSION-1.el7.x86_64.rpm/download)
 [RPM RHEL 8/CentOS 8](https://packagecloud.io/github/git-lfs/packages/el/8/git-lfs-VERSION-1.el8.x86_64.rpm/download)
 [Debian 8](https://packagecloud.io/github/git-lfs/packages/debian/jessie/git-lfs_VERSION_amd64.deb/download)