remove support for Ubuntu 18.04 #400

Job	Run time
Vim v8.1.2424 on macOS	2m 20s
Neovim v0.4.4 (ubuntu-latest)	23s
Vim v8.2.1931 (ubuntu-latest)	1m 3s
Neovim v0.4.4 (macos-latest)	32s
Stable and nightly (ubuntu-latest, stable, true)	23s
Vim v8.2.1931 (macos-latest)	1m 13s
Neovim v0.4.4 (windows-latest)	1m 7s
Stable and nightly (ubuntu-latest, stable, false)	30s
Vim v8.2.1931 (windows-latest)	1m 43s
Check unit tests and linter (ubuntu-latest)	37s
Stable and nightly (ubuntu-latest, nightly, true)	27s
Check unit tests and linter (macos-latest)	27s
Stable and nightly (ubuntu-latest, nightly, false)	1m 11s
Check unit tests and linter (macos-12)	1m 22s
Stable and nightly (macos-latest, stable, true)	27s
Stable and nightly (macos-latest, stable, false)	54s
Stable and nightly (macos-latest, nightly, true)	17s
Stable and nightly (macos-latest, nightly, false)	1m 16s
Stable and nightly (windows-latest, stable, true)	56s
Stable and nightly (windows-latest, stable, false)	1m 32s
Stable and nightly (windows-latest, nightly, true)	1m 11s
Stable and nightly (windows-latest, nightly, false)	1m 19s
Stable and nightly (macos-12, stable, true)	51s
Stable and nightly (macos-12, stable, false)	2m 16s
Stable and nightly (macos-12, nightly, true)	47s
Stable and nightly (macos-12, nightly, false)	2m 41s
	27m 45s

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