feat: add scaletargetref exists check in webhook and revise some variable name

[ctccxxd]

check scaletargetref exists in webhook and revise some variable name, originally,
1, There is not scaletargetref exists check code in webhook module.
2, Change the inappropriate variable name Gckr to Gvkr.
3, If still want to remain the function to create scaleobj first without real scaletargetRef obj. Think whether this function will be a way can be configged, to confirm the real scaletargetRef obj already exists when we create/update scaleobj. In this way, it can avoid the user misconfigurations about scaletargetRef in yaml file.

Checklist

• When introducing a new scaler, I agree with the scaling governance policy
• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added
• Changelog has been updated and is aligned with our changelog requirements
• A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #

Relates to #
#6354

[SpiritZhou]

Could you please open an issue for this improvement and relate this pr to the issue? This will make it clear and easy to tracing.

[ctccxxd]

Could you please open an issue for this improvement and relate this pr to the issue? This will make it clear and easy to tracing.

done. Thank you.

[ctccxxd]

Friendly ping @zroubalik @wozniakjan @SpiritZhou @psi @wonko @fivesheep @JorTurFer hope you to review and discuss, much thanks.