assertions: correct the type assertion of ErrorAs

[shoenig]

After bumping the go directive version in go.mod to 1.21 the vet tool
is complaining about the ErrorAs assertion.

➜ go vet ./...
internal/assertions/assertions.go:208:6: second argument to errors.As must be a non-nil pointer to either a type that implements error, or to any interface type

The linter warning is a bit of a red-herring - in fact the Target
parameter for our ErrorAs assertion has been incorrect this whole time.
It should be any. Currently it is restricting types to be a concrete
type that implements the error interface, which is not a requirement of
errors.As.

In fact you can use errors.As to assert your err satisfies any interface
you choose - but the examples in the Go docs do not make this clear (imo).

For example you could have some error type:

type NetworkError interface {
    Error() error
    Timeout() bool
}

And you want to check if your error implements Timeout(), which you
can do using errors.As, e.g.

type IO interface {
    Timeout() bool
}

var io IO
if errors.As(networkError, &io) {
    timeout := io.Timeout()
    // ...
}

In this toy example we use errors.As detect if the underlying error
(which is NetworkError) satisfies the IO interface, and if so, populates
io with our error value, and call its Timeout() method.

[alicebob]

This made me cleanup a bunch of ErrorAs into ErrorIs checks, which is what they should have been in the first place.