Fix: Have single source of truth for ICR urls

[bainsy88]

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Closes #6737

This PR changes the behaviour to constructing the private ICR endpoint based on the public one. This is deterministic and means that it is only maintained in one place.

Output from acceptance testing:

$ make testacc TEST=./ibm/service/registry TESTARGS='-run="TestAccIBMCr.*"'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./ibm/service/registry -v -run="TestAccIBMCr.*" -timeout 700m 
=== RUN   TestAccIBMCrNamespacesDataSourceBasic
--- PASS: TestAccIBMCrNamespacesDataSourceBasic (37.75s)
=== RUN   TestAccIBMCrNamespaceBasic
--- PASS: TestAccIBMCrNamespaceBasic (46.64s)
=== RUN   TestAccIBMCrNamespaceAllArgs
--- PASS: TestAccIBMCrNamespaceAllArgs (69.03s)
=== RUN   TestAccIBMCrRetentionPolicyAllArgs
--- PASS: TestAccIBMCrRetentionPolicyAllArgs (38.95s)
PASS