Update to Go 1.20

[andrerfcsantos]

Description

This PR prepares the main repo for the track upgrade to Go 1.20:

• Updated CI to run Go 1.20
• Set default version to 1.18 in the config file of gomod-sync
  • This means CI will also now require new exercises to have a go.mod set to 1.18
• Updated all go.mod files to 1.18 (gomod-sync update)
• Removed two notes mentioning the track is on Go 1.16
• Updated golanci-lint to the most recent version

Related

PR that also should be merged for the track to be considered "upgraded":

• Update test runner: Update to Go 1.20 go-test-runner#97

This PR should be merged eventually, but there's no rush since Go currently doesn't have the representer or the analyzer enabled:

• Update representer: Update to Go 1.20 go-representer#34
• Update analyzer: Update to Go 1.20 go-analyzer#40

[andrerfcsantos]

Before we merge this, I think we should also add go.mod to the files.invalidator of the exercises, what do you think @junedev?

A change to the version of the go.mod is not as drastic as the equivalent in other languages, and if students have the old 1.16 go.mod, their solution will work with the test generator. But it will limit the features they can use (unless they update + submit the go.mod themselves) and I'm not sure there's other way to notify them the go.mod changed other than prompting for an exercise update.

cc @ErikSchierboom

[ErikSchierboom]

Before we merge this, I think we should also add go.mod to the files.invalidator of the exercises

I know you didn't ask me, but: yes! This would be really helpful to students.