chore(docs): remove gopkg.in link

Using gopkg.in doesn't work for versions >1.20.1 as once a `go.mod` file
exists, the go tooling enforces that the import path must match.

Fixes #1510

Signed-off-by: Dominic Evans <dominic.evans@uk.ibm.com>

README.md

@@ -19,8 +19,9 @@ Sarama provides a "2 releases + 2 months" compatibility guarantee: we support
 the two latest stable releases of Kafka and Go, and we provide a two month
 grace period for older releases. However, older releases of Kafka are still likely to work.
 
-Sarama follows semantic versioning and provides API stability via the gopkg.in service.
-You can import a version with a guaranteed stable API via http://gopkg.in/IBM/sarama.v1.
+Sarama follows semantic versioning and provides API stability via the standard Go
+[module version numbering](https://go.dev/doc/modules/version-numbers) scheme.
+
 A changelog is available [here](CHANGELOG.md).
 
 ## Contributing