chore: Relax the GKE WIF principal:// restriction

[benjamin-maynard]

Submit once cl/749106195 is in prod.

Relax GKE WIF limitation. b/385138184.

Running tool: /usr/local/go/bin/go test -timeout 30s -run ^(TestNewMechanismWithTokenSource|TestMechanism_Name|TestMechanism_Start)$ github.com/googleapis/managedkafka/sasl-plain-access-token/segmentio/saslplainoauthmechanism

=== RUN   TestNewMechanismWithTokenSource
=== RUN   TestNewMechanismWithTokenSource/Valid_Token_and_Email
--- PASS: TestNewMechanismWithTokenSource/Valid_Token_and_Email (0.00s)
=== RUN   TestNewMechanismWithTokenSource/Valid_Token,_Bad_Email
--- PASS: TestNewMechanismWithTokenSource/Valid_Token,_Bad_Email (0.00s)
=== RUN   TestNewMechanismWithTokenSource/Token_Error
--- PASS: TestNewMechanismWithTokenSource/Token_Error (0.00s)
--- PASS: TestNewMechanismWithTokenSource (0.00s)
=== RUN   TestMechanism_Name
--- PASS: TestMechanism_Name (0.00s)
=== RUN   TestMechanism_Start
=== RUN   TestMechanism_Start/Valid_Token_and_Email
--- PASS: TestMechanism_Start/Valid_Token_and_Email (0.00s)
--- PASS: TestMechanism_Start (0.00s)
PASS
ok      github.com/googleapis/managedkafka/sasl-plain-access-token/segmentio/saslplainoauthmechanism    0.496s