CONTRIBUTING.md: update links

- docs moved to https://go.dev/doc/
- blog moved to https://go.dev/blog/
- update DCO link to use https

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

CONTRIBUTING.md

@@ -192,7 +192,7 @@ For more details, see the [MAINTAINERS](MAINTAINERS) page.
 The sign-off is a simple line at the end of the explanation for the patch. Your
 signature certifies that you wrote the patch or otherwise have the right to pass
 it on as an open-source patch. The rules are pretty simple: if you can certify
-the below (from [developercertificate.org](http://developercertificate.org/)):
+the below (from [developercertificate.org](https://developercertificate.org):
 
 ```
 Developer Certificate of Origin
@@ -336,9 +336,8 @@ The rules:
 1. All code should be formatted with `gofumpt` (preferred) or `gofmt -s`.
 2. All code should pass the default levels of
    [`golint`](https://github.com/golang/lint).
-3. All code should follow the guidelines covered in [Effective
-   Go](http://golang.org/doc/effective_go.html) and [Go Code Review
-   Comments](https://github.com/golang/go/wiki/CodeReviewComments).
+3. All code should follow the guidelines covered in [Effective Go](https://go.dev/doc/effective_go)
+   and [Go Code Review Comments](https://github.com/golang/go/wiki/CodeReviewComments).
 4. Comment the code. Tell us the why, the history and the context.
 5. Document _all_ declarations and methods, even private ones. Declare
    expectations, caveats and anything else that may be important. If a type
@@ -360,6 +359,6 @@ The rules:
     guidelines. Since you've read all the rules, you now know that.
 
 If you are having trouble getting into the mood of idiomatic Go, we recommend
-reading through [Effective Go](https://golang.org/doc/effective_go.html). The
-[Go Blog](https://blog.golang.org) is also a great resource. Drinking the
+reading through [Effective Go](https://go.dev/doc/effective_go). The
+[Go Blog](https://go.dev/blog/) is also a great resource. Drinking the
 kool-aid is a lot easier than going thirsty.