fix: mark existing_kms_instance_crn as required instead of existing_kms_key_crn in the DA

[ocofaigh]

Description

• mark existing_kms_instance_crn as required instead of existing_kms_key_crn in the DA because when a value is passed for existing_kms_instance_crn it will create the root key if no value is passed for existing_kms_key_crn.
• Updated some of the KMS variable descriptions in the DA

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/run pipeline

[terraform-ibm-modules-ops]

🎉 This PR is included in version 1.23.5 🎉

The release is available on:

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• v1.23.5

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