Add stronger warning against attempting a shared tier upgrade

[themantissa]

Description

Users who attempt to upgrade from a shared tier cluster (M2/M5) to a dedicated tier cluster - even though it is called out as unsupported - will destroy the shared tier and create a dedicated tier. This is due to the fact that upgrades are not supported yet Terraform will still try to make the changes it sees in the config. Adding stronger language to the call out that this is not supported since it will result in data lost from the shared tier cluster.

Link to any related issue(s):

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)
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the Terraform contribution guidelines
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code

Further comments

[threebee]

LGTM!