chore: drop support for Ubuntu 16.04

As of April 2021, it will no longer be an LTS release, and thus us supporting it will increase maintenance burdens.

Also switch CI to use 18.04, so as to test what we currently support.

Closes #4505

bootstrap.sh

@@ -3,7 +3,7 @@ set -eo pipefail
 [[ $TRACE ]] && set -x
 
 # A script to bootstrap dokku.
-# It expects to be run on Ubuntu 16.04/18.04/20.04, or CentOS 7 via 'sudo'
+# It expects to be run on Ubuntu 18.04/20.04, or CentOS 7 via 'sudo'
 # If installing a tag higher than 0.3.13, it may install dokku via a package (so long as the package is higher than 0.3.13)
 # It checks out the dokku source code from Github into ~/dokku and then runs 'make install' from dokku source.
 
@@ -12,7 +12,7 @@ set -eo pipefail
 # That's good because it prevents our output overlapping with wget's.
 # It also means that we can't run a partially downloaded script.
 
-SUPPORTED_VERSIONS="Debian [9, 10], CentOS [7], Fedora (partial) [33, 34], Ubuntu [16.04, 18.04, 20.04]"
+SUPPORTED_VERSIONS="Debian [9, 10], CentOS [7], Fedora (partial) [33, 34], Ubuntu [18.04, 20.04]"
 
 log-fail() {
   declare desc="log fail formatter"
@@ -155,7 +155,7 @@ install-dokku-from-deb-package() {
   local NO_INSTALL_RECOMMENDS=${DOKKU_NO_INSTALL_RECOMMENDS:=""}
   local OS_ID
 
-  if ! in-array "$DOKKU_DISTRO_VERSION" "16.04" "18.04" "20.04" "9" "10"; then
+  if ! in-array "$DOKKU_DISTRO_VERSION" "18.04" "20.04" "9" "10"; then
     log-fail "Unsupported Linux distribution. Only the following versions are supported: $SUPPORTED_VERSIONS"
   fi
 
@@ -187,7 +187,7 @@ install-dokku-from-deb-package() {
   fi
 
   if [[ "$DOKKU_DISTRO" == "ubuntu" ]]; then
-    OS_IDS=("xenial" "bionic" "focal")
+    OS_IDS=("bionic" "focal")
     if ! in-array "$OS_ID" "${OS_IDS[@]}"; then
       OS_ID="bionic"
     fi