Update go version to 1.21

[gold-kou]

Description

The previous PR for updating go was reverted since the ci failed.
#107

This PR is a revenge of it.
I added to fix the version of sigs.k8s.io/kustomize/kustomize from [email protected] to [email protected] which is the latest.
I assume that the old kustomize used exclude directive is a reason.
https://github.com/kubernetes-sigs/kustomize/blob/kustomize/v3.8.7/kustomize/go.mod

Confirmed the make command works correctly.

% make manifests-release
/Users/koki.hatano/github/gold-kou/gatling-operator/bin/controller-gen "crd:trivialVersions=true,preserveUnknownFields=false" rbac:roleName=manager-role webhook paths="./..." output:crd:artifacts:config=config/crd/bases
cd config/manager && /Users/koki.hatano/github/gold-kou/gatling-operator/bin/kustomize edit set image controller=gatling-operator:20240301-143405
/Users/koki.hatano/github/gold-kou/gatling-operator/bin/kustomize build config/default > gatling-operator.yaml

The other changes are same with #105 .

Checklist

Please check if applicable

• Tests have been added (if applicable, ie. when operator codes are added or modified)
• Relevant docs have been added or modified (if applicable, ie. when new features are added or current features are modified)

Relevant issue #

[sejima1105]

LGTM!