esVersion support both int or str

[NissesSenap]

Description

Use intOrStr on esVersion in the datasource to be compatible with both grafana 7 and 8.
The only thing that have been changed in the GrafanaDataSourceJsonData struct is the esVersion, the rest is just indentation.

Fixes #753

Relevant issues/tickets

#753

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• This change requires a documentation update
• I have added tests that prove my fix is effective or that my feature works
• I have added a test case that will be used to verify my changes
• Verified independently on a cluster by reviewer

Verification steps

make run
k apply -f deploy/examples/datasources/es.yaml

port-forward to the deployment and verify that the datasource is getting the version set.

[NissesSenap]

ping @faelin and @redradrat

[hubeadmin]

LGTM, thanks! :)