fix: update validation for cloud_monitoring_instance_crn variable (#290)

[HendrikSchrieber]

Description

Closes #290

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[ocofaigh]

Thanks for the fix!

[ocofaigh]

/run pipeline

[ocofaigh]

Test failed with:

│ Error: ---
│ id: terraform-86e3f8de
│ summary: 'CreateProfileWithContext failed: Service Unavailable'
│ severity: error
│ resource: ibm_iam_trusted_profile
│ operation: create
│ component:
│   name: github.com/IBM-Cloud/terraform-provider-ibm
│   version: 1.80.3
│ ---

Unrelated to the changes in this PR so re-running..

[ocofaigh]

/run pipeline

[ocofaigh]

/run pipeline

[terraform-ibm-modules-ops]

🎉 This PR is included in version 1.10.12 🎉

The release is available on:

• GitHub release
• v1.10.12

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