18.04 -> 20.04

Author: jyn514

.github/ISSUE_TEMPLATE/missing-linux-packages.md

@@ -8,9 +8,9 @@ assignees: ''
 ---
 
 <!-- Thanks for reporting this issue!
-     Please remember our build environment is based on Ubuntu 18.04 LTS,
+     Please remember our build environment is based on Ubuntu 20.04 LTS,
      so the packages you're requesting need to be available in its
      repositories. -->
 
 **Affected crates:** foo, foo-sys
-**Ubuntu 18.04 packages to install:** libfoo-dev
+**Ubuntu 20.04 packages to install:** libfoo-dev

README.md

@@ -1,7 +1,7 @@
 # Build environment for Rust crates
 
 This repository contains the source of the Docker container the Rust project
-uses to build third-party crates. It is based on **Ubuntu 18.04**, and contains
+uses to build third-party crates. It is based on **Ubuntu 20.04**, and contains
 all the native dependencies used by the Rust crates we know of.
 
 ## Adding new dependencies