`civo_template` resource should be removed and `civo_template` data source should be deprecated

[zulh-civo]

In line with legacy/OpenStack EOL where Civo customers can no longer perform CRUD on Templates (this was replaced by the new Disk Images), we should:

• Remove civo_template resource and its documentation
• Deprecate civo_template data source and update the documentation
• Add new civo_disk_image data source, together with documentation
• Show warning when user uses template attribute in civo_instance resource
• Add new disk_image attribute in civo_instance resource
• When user uses both template and disk_image attributes in civo_instance resource, raise an error
• Update civo_instance resource documentation