fix: allow grpc server enabled even when api off

[julienrbrt]

Description

I wondered why #14888 needed to enable the API server, while it was needing only the GRPC server.
Turns out this PR #14652 added a regression, where the gRPC server was only started when the API server is (as gRPC-Web and gRPC-Gateway are on the same port, we need a gRPC instance there).

This PR adds a condition to still start the gRPC server even when the API is disabled.
We can then revert #14888 as well.

No changelog required because we are on main and this was never released.

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