provider/azurerm: Network Security Group - ignoring protocol casing at Import time

[tombuildsstuff]

Fixes #13085 - when importing a Network Security Group, the Protocol this PR ignores the casing on the Protocol field

Tests pass:

 $ envchain azurerm make testacc TEST=./builtin/providers/azurerm/ TESTARGS='-run=TestAccAzureRMNetworkSecurityGroup'
TF_ACC=1 go test ./builtin/providers/azurerm/ -v -run=TestAccAzureRMNetworkSecurityGroup -timeout 120m
=== RUN   TestAccAzureRMNetworkSecurityGroup_importBasic
--- PASS: TestAccAzureRMNetworkSecurityGroup_importBasic (113.06s)
=== RUN   TestAccAzureRMNetworkSecurityGroup_basic
--- PASS: TestAccAzureRMNetworkSecurityGroup_basic (109.59s)
=== RUN   TestAccAzureRMNetworkSecurityGroup_disappears
--- PASS: TestAccAzureRMNetworkSecurityGroup_disappears (107.92s)
=== RUN   TestAccAzureRMNetworkSecurityGroup_withTags
--- PASS: TestAccAzureRMNetworkSecurityGroup_withTags (136.80s)
=== RUN   TestAccAzureRMNetworkSecurityGroup_addingExtraRules
--- PASS: TestAccAzureRMNetworkSecurityGroup_addingExtraRules (144.80s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/azurerm	612.193s

[amcguign]

Thanks Tom, appreciate the super-quick turnaround on this little one!

[ghost]

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