Updating the Go version in go.mod

At this time both Go 1.19 and 1.20 are supported. The version
specified in the go.mod file is the minimum version we expect Helm
to be compiled against. This is the oldest supported version to
support environments where others compile Helm. The Helm project
is using Go 1.20 to build Helm itself.

Updating to Go 1.19 also includes dealing with io/ioutil
deprecation and some additional linting issues around staticcheck.
All the staticcheck issues were in test files so linting was
skipped for those.

Signed-off-by: Matt Farina <matt.farina@suse.com>