fix: unified scim metadata key casing (#9244)

# Which Problems Are Solved
- SCIM user metadata mapping keys have differing case styles.

# How the Problems Are Solved
- key casing style is unified to strict camelCase

# Additional Context
Part of #8140

Although this is technically a breaking change, it is considered
acceptable because the SCIM feature is still in the preview stage and
not fully implemented yet.

Co-authored-by: Stefan Benz <46600784+stebenz@users.noreply.github.com>

internal/api/scim/integration_test/users_replace_test.go

@@ -296,7 +296,7 @@ func TestReplaceUser_scopedExternalID(t *testing.T) {
 	// set provisioning domain of service user
 	_, err = Instance.Client.Mgmt.SetUserMetadata(CTX, &management.SetUserMetadataRequest{
 		Id:    Instance.Users.Get(integration.UserTypeOrgOwner).ID,
-		Key:   "urn:zitadel:scim:provisioning_domain",
+		Key:   "urn:zitadel:scim:provisioningDomain",
 		Value: []byte("fooBazz"),
 	})
 	require.NoError(t, err)
@@ -327,7 +327,7 @@ func TestReplaceUser_scopedExternalID(t *testing.T) {
 
 	_, err = Instance.Client.Mgmt.RemoveUserMetadata(CTX, &management.RemoveUserMetadataRequest{
 		Id:  Instance.Users.Get(integration.UserTypeOrgOwner).ID,
-		Key: "urn:zitadel:scim:provisioning_domain",
+		Key: "urn:zitadel:scim:provisioningDomain",
 	})
 	require.NoError(t, err)
 }