remove support for Ubuntu 18.04

ubuntu-18.04 GH-hosted runner was already dropped:

https://github.blog/changelog/2022-08-09-github-actions-the-ubuntu-18-04-actions-runner-image-is-being-deprecated-and-will-be-removed-by-12-1-22/

Ubuntu 18.04 is no longer maintained:

https://ubuntu.com/blog/18-04-end-of-standard-support

package.json

@@ -48,8 +48,7 @@
     "@actions/github": "^6.0.0",
     "@actions/io": "^1.1.3",
     "node-fetch": "^2.6.7",
-    "shlex": "^2.1.2",
-    "ubuntu-version": "^2.0.0"
+    "shlex": "^2.1.2"
   },
   "devDependencies": {
     "@types/eslint__js": "^8.42.3",

src/install_linux.ts

@@ -1,28 +1,14 @@
 import * as core from '@actions/core';
-import { getUbuntuVersion } from 'ubuntu-version';
 import type { Installed } from './install';
 import type { Config } from './config';
 import { exec } from './shell';
 import { buildVim } from './vim';
 import { buildNightlyNeovim, downloadNeovim, downloadStableNeovim } from './neovim';
 
-async function isUbuntu18OrEarlier(): Promise<boolean> {
-    const version = await getUbuntuVersion();
-    if (version.length === 0) {
-        core.error('Trying to install apt package but current OS is not Ubuntu');
-        return false; // Should be unreachable
-    }
-
-    core.debug(`Ubuntu system version: ${version.join('.')}`);
-
-    return version[0] <= 18;
-}
-
 async function installVimStable(): Promise<Installed> {
     core.debug('Installing stable Vim on Linux using apt');
-    const pkg = (await isUbuntu18OrEarlier()) ? 'vim-gnome' : 'vim-gtk3';
     await exec('sudo', ['apt-get', 'update', '-y']);
-    await exec('sudo', ['apt-get', 'install', '-y', '--no-install-recommends', pkg]);
+    await exec('sudo', ['apt-get', 'install', '-y', '--no-install-recommends', 'vim-gtk3']);
     return {
         executable: 'vim',
         binDir: '/usr/bin',

test/vim.ts

@@ -201,44 +201,14 @@ describe('installVimStable()', function () {
 
     before(function () {
         stub = mockExec();
+        installVimOnLinux = mock.reRequire('../src/install_linux').install;
     });
 
     after(function () {
         mock.stop('../src/shell');
     });
 
-    afterEach(function () {
-        mock.stop('ubuntu-version');
-    });
-
-    it('installs vim-gnome package for Ubuntu 18.10 or earlier (#11)', async function () {
-        async function getUbuntuVersion(): Promise<unknown> {
-            return Promise.resolve([18, 4]);
-        }
-        mock('ubuntu-version', { getUbuntuVersion });
-        installVimOnLinux = mock.reRequire('../src/install_linux').install;
-
-        const installed = await installVimOnLinux({
-            version: 'stable',
-            neovim: false,
-            os: 'linux',
-            configureArgs: null,
-            token: null,
-        });
-
-        A.equal(installed.executable, 'vim');
-        A.equal(installed.binDir, '/usr/bin');
-        const aptArgs = stub.called[stub.called.length - 1][1];
-        A.equal(aptArgs[aptArgs.length - 1], 'vim-gnome', aptArgs.join(' '));
-    });
-
-    it('installs vim-gtk3 package for Ubuntu 19.04 or later (#11)', async function () {
-        async function getUbuntuVersion(): Promise<unknown> {
-            return Promise.resolve([20, 4, 2]);
-        }
-        mock('ubuntu-version', { getUbuntuVersion });
-        installVimOnLinux = mock.reRequire('../src/install_linux').install;
-
+    it('installs vim-gtk3 package', async function () {
         const installed = await installVimOnLinux({
             version: 'stable',
             neovim: false,