fix: set min-go-version to 1.16

Changes that were introduced in 8ccea19
made it impossible to compile sarama with go 1.15
(which is mentioned as officially supported version in the README file)

io.Discard and io.ReadAll appeared only in 1.16
https://golang.org/doc/go1.16#ioutil

To address this issue this commit bumps minimal supported go version
defined in go.mod file, to provide a clear message that 1.16 is required

```
> go build
./config.go:678:37: undefined: io.Discard
./decompress.go:43:10: undefined: io.ReadAll
./decompress.go:55:10: undefined: io.ReadAll
./sarama.go:89:29: undefined: io.Discard
note: module requires Go 1.16
```

CHANGELOG.md

@@ -6,6 +6,10 @@
 
 - #1912 - @faillefer Support for --delete-offsets for consumer group topic
 
+# Fixes
+
+- #2048 - @troyanov - Set minimum supported go version to 1.16
+
 #### Version 1.28.0 (2021-02-15)
 
 **Note that with this release we change `RoundRobinBalancer` strategy to match Java client behavior. See #1788 for details.**

README.md

@@ -19,7 +19,7 @@ You might also want to look at the [Frequently Asked Questions](https://github.c
 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. This means we currently officially support
-Go 1.15 through 1.16, and Kafka 2.7 through 2.8, although older releases are
+Go 1.16, and Kafka 2.7 through 2.8, although older releases are
 still likely to work.
 
 Sarama follows semantic versioning and provides API stability via the gopkg.in service.

go.mod

@@ -1,6 +1,6 @@
 module github.com/Shopify/sarama
 
-go 1.13
+go 1.16
 
 require (
 	github.com/Shopify/toxiproxy/v2 v2.1.6-0.20210914104332-15ea381dcdae