Mariner 2.0 stable toolkit now needs golang-1.20 in Ubuntu 18

[Pravila00]

After the latest release of Mariner2.0-stable, today my build of the Mariner 2.0 toolkit using Ubuntu stopped working properly. It seems that now some packages from the toolkit are using golang-1.20 features.

For example, imagecustomizerlib now uses filepath.IsLocal which is only added after golang-1.20. These are the logs when building the toolkit:

# github.com/microsoft/CBL-Mariner/toolkit/tools/pkg/imagecustomizerlib
pkg/imagecustomizerlib/imagecustomizer.go:149:15: undefined: filepath.IsLocal
pkg/imagecustomizerlib/imagecustomizer.go:328:34: undefined: strings.CutPrefix

I believe the requirements for Ubuntu18 should be updated at prerrequisites-ubuntu.md, as now golang-1.20 is needed.

[christopherco]

Thanks for highlighting this @Pravila00 and sorry about the disruption. We had the doc changes prepared (aafec95) but looks like they had missed the 2.0-stable release train.

Tagging @PawelWMS, @cwize1 , @eric-desrochers for awareness

In general, we recommend updating to a Ubuntu 22.04 or Mariner 2.0 environment since Ubuntu 18.04 is EOL. Additionally, golang 1.19 recently went EOL as well, so it is recommended to move forward to golang 1.20