Stop building the "x86-64-unknown-linux-gnu" ponyc package

The "x86-64-unknown-linux-gnu" package was intended to be "generic", however, it
is not actually generic. It is a glibc version that is built on "some glibc
using Linux" and will work if it is installed on a Linux that is compatible at
a library level withthe system that we built the package on.

Quite some time ago, Theo and I (who made the initial decision to include
a "x86-64-unknown-linux-gnu" package) realized that the
 "x86-64-unknown-linux-gnu" package idea was "a very bad idea".

There are a couple large problems with this approach:

1- anyone using the pacakge on glibc distro that isn't library compatible with
our "x86-64-unknown-linux-gnu" build will have the installed ponyc fail in
interesting and confounding ways. That's a bad user experience.

2- in order to not fall horribly behind, from time to time, we need to
"rev" the environment used to build our "x86-64-unknown-linux-gnu" packages.
For us that has meant changing the Ubuntu version periodically. That then means
that people who had an environment that worked will do an update and install
a ponyc that doesn't work because it is for a different set of libraries.
That's a bad user experience as well.

This PR drops "x86-64-unknown-linux-gnu" from being built and mirrors a change
in ponyup that dropped support for using the package via ponyup. See
ponylang/ponyup#245 for more information about the
ponyup change.

.ci-dockerfiles/cross-aarch64/Dockerfile

@@ -1,4 +1,4 @@
-FROM ponylang/ponyc-ci-x86-64-unknown-linux-gnu-builder:20230115
+FROM ponylang/ponyc-ci-x86-64-unknown-linux-ubuntu20.04-builder:20230115
 
 ARG CROSS_TRIPLE=aarch64-unknown-linux-gnu
 ARG CROSS_CC=aarch64-linux-gnu-gcc

.ci-dockerfiles/cross-arm/Dockerfile

@@ -1,4 +1,4 @@
-FROM ponylang/ponyc-ci-x86-64-unknown-linux-gnu-builder:20230115
+FROM ponylang/ponyc-ci-x86-64-unknown-linux-ubuntu20.04-builder:20230115
 
 ARG CROSS_TRIPLE=arm-unknown-linux-gnueabi
 ARG CROSS_CC=arm-linux-gnueabi-gcc

.ci-dockerfiles/cross-armhf/Dockerfile

@@ -1,4 +1,4 @@
-FROM ponylang/ponyc-ci-x86-64-unknown-linux-gnu-builder:20230115
+FROM ponylang/ponyc-ci-x86-64-unknown-linux-ubuntu20.04-builder:20230115
 
 ARG CROSS_TRIPLE=arm-unknown-linux-gnueabihf
 ARG CROSS_CC=arm-linux-gnueabihf-gcc

.ci-dockerfiles/cross-riscv64/Dockerfile

@@ -1,4 +1,4 @@
-FROM ponylang/ponyc-ci-x86-64-unknown-linux-gnu-builder:20230115
+FROM ponylang/ponyc-ci-x86-64-unknown-linux-ubuntu20.04-builder:20230115
 
 USER root
 

.cirrus.yml

@@ -288,13 +288,6 @@ task:
   timeout_in: 120m
 
   matrix:
-    - name: "nightly: x86-64-unknown-linux-gnu"
-      container:
-        image: ponylang/ponyc-ci-x86-64-unknown-linux-gnu-builder:20230115
-      environment:
-        IMAGE: ponylang/ponyc-ci-x86-64-unknown-linux-gnu-builder:20230115
-        TRIPLE_VENDOR: unknown
-        TRIPLE_OS: linux-gnu
     - name: "nightly: x86-64-unknown-linux-ubuntu18.04"
       container:
         image: ponylang/ponyc-ci-x86-64-unknown-linux-ubuntu18.04-builder:20230115
@@ -450,13 +443,6 @@ task:
   timeout_in: 120m
 
   matrix:
-    - name: "release: x86-64-unknown-linux-gnu"
-      container:
-        image: ponylang/ponyc-ci-x86-64-unknown-linux-gnu-builder:20230115
-      environment:
-        IMAGE: ponylang/ponyc-ci-x86-64-unknown-linux-gnu-builder:20230115
-        TRIPLE_VENDOR: unknown
-        TRIPLE_OS: linux-gnu
     - name: "release: x86-64-unknown-linux-ubuntu18.04"
       container:
         image: ponylang/ponyc-ci-x86-64-unknown-linux-ubuntu18.04-builder:20230115

.release-notes/no-more-gnu.md

@@ -0,0 +1,5 @@
+## Stop building "x86-64-unknown-linux-gnu" packages
+
+Previously we built a "generic gnu" ponyc package and made it available via Cloudsmith and ponyup. Unfortunately, there is no such thing as a "generic gnu" ponyc build that is universally usable. The ponyc package only worked if the library versions on your Glibc based Linux distribution matched those of our build environment.
+
+We've stoped building the "generic gnu" aka "x86-64-unknown-linux-gnu" packages of ponyc as they had limited utility and a couple of different ways of creating a very bad user experience. Please see our [Linux installation instructions](https://github.com/ponylang/ponyc/blob/main/INSTALL.md#linux) for a list of Linux distributions we currently create ponyc packages for.