Migrate build workflow to GoReleaser and GitHub CI

[sid-maddy]

Changes

Migrate build workflow to GoReleaser and GitHub CI

This makes releasing updates to Krew easier. Also removed the Makefile, since it seems like most of its tasks are now done by GoReleaser. Let me know if I should add it back.

Sample run (without the krew-index PR creation step enabled) and associated release

Closes https://github.com/emirozer/kubectl-doctor/issues/18

Update Go version to a supported version

As per Go's release policy,

Each major Go release is supported until there are two newer major releases.

The major release being used is 1.12, which has reached EOL. The current major release, as of this PR, is 1.19, which will be supported till 1.21 is released.

Add instructions for installing using Krew

Closes https://github.com/emirozer/kubectl-doctor/issues/17