vault_mfa_duo path and arguments fix

[tobyrounds]

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Relates #1468

Release note for CHANGELOG:

Updated vault_mfa_duo to support correct API endpoint and arguments in Vault OSS per [1.10 GA release](https://www.hashicorp.com/blog/login-mfa-support-added-to-vault-open-source-and-hcp-vault)

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestMFADuoBasic'

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test -v -v --run TestMFADuoBasic -timeout 30m ./...
?   	github.com/hashicorp/terraform-provider-vault	[no test files]
?   	github.com/hashicorp/terraform-provider-vault/cmd/coverage	[no test files]
?   	github.com/hashicorp/terraform-provider-vault/cmd/generate	[no test files]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/codegen	0.115s [no tests to run]
?   	github.com/hashicorp/terraform-provider-vault/generated	[no test files]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/generated/datasources/transform/decode	0.390s [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/generated/datasources/transform/encode	0.241s [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/alphabet	0.560s [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/role	0.713s [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/template	0.839s [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/transformation	0.983s [no tests to run]
?   	github.com/hashicorp/terraform-provider-vault/helper	[no test files]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/internal/identity/entity	0.126s [no tests to run]
?   	github.com/hashicorp/terraform-provider-vault/schema	[no test files]
?   	github.com/hashicorp/terraform-provider-vault/testutil	[no test files]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/util	0.214s [no tests to run]
=== RUN   TestMFADuoBasic
--- PASS: TestMFADuoBasic (2.24s)
PASS
ok  	github.com/hashicorp/terraform-provider-vault/vault	2.517s
...

[hashicorp-cla]

All committers have signed the CLA.

[xueshanf]

Vault release 1.11.0 removed legacy MFA duo support. It would be really nice to have this fixed so we can migrate over. Thanks!