Fix updating of `go.mod` entries for components

We noticed that some of our components had not received `go.mod` updates
while they did receive updates for the versions declared in the YAML
manifests.

Was able to trace this back to a behavior change in Go since `1.16.x`,
resulting in it no longer making automated changes to `go.mod` and
`go.sum`[1]. This is an issue for our updater script as it relies
on `go list -m all`, which now after the first `go mod edit` returns:

```console
$ go list -m all
go: github.com/fluxcd/notification-controller/api@v0.10.0: missing
go.sum entry; to add it:
        go mod download github.com/fluxcd/notification-controller/api
```

To work around the issue without having to repeatedly call `go mod
tidy`, I have opted to simply `grep` on the contents of `go.mod` as a
workaround.

[1]: https://blog.golang.org/go116-module-changes#TOC_3

.

Signed-off-by: Hidde Beydals <hello@hidde.co>