chore(auth): fix staticcheck errors

[julieqiu]

Fix these issues:

credentials/detect.go:102:12: metadata.ProjectID is deprecated: Please use the context aware variant [ProjectIDWithContext].  (SA1019)
credentials/idtoken/compute.go:47:11: metadata.ProjectID is deprecated: Please use the context aware variant [ProjectIDWithContext].  (SA1019)
grpctransport/grpctransport.go:294:9: grpc.DialContext is deprecated: use NewClient instead.  Will be supported throughout 1.x.  (SA1019)
grpctransport/pool_test.go:71:16: grpc.Dial is deprecated: use NewClient instead.  Will be supported throughout 1.x.  (SA1019)
internal/transport/s2a.go:87:9: metadata.Get is deprecated: Please use the context aware variant [GetWithContext].  (SA1019)

For #10779

[erezrokah]

Hello 👋 FYI we're discussing this change in grpc/grpc-go#7556 (comment). The change from DialContext to NewClient also changes the schema from passthrough to DNS (see https://github.com/grpc/grpc-go/blob/master/Documentation/anti-patterns.md#the-wrong-way-grpcdial) which is breaking for some clients that use this SDK

[erezrokah]

Opened a bug report #11089