chore: drop support for Go 1.15

Signed-off-by: Mark Sagi-Kazar <mark.sagikazar@gmail.com>

.github/workflows/ci.yaml

@@ -16,7 +16,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        go: ['1.15', '1.16', '1.17', '1.18', '1.19']
+        go: ['1.16', '1.17', '1.18', '1.19']
         tags: ['', 'viper_yaml2', 'viper_toml1']
     env:
       GOFLAGS: -mod=readonly

README.md

@@ -11,7 +11,7 @@
 [![GitHub Workflow Status](https://img.shields.io/github/workflow/status/spf13/viper/CI?style=flat-square)](https://github.com/spf13/viper/actions?query=workflow%3ACI)
 [![Join the chat at https://gitter.im/spf13/viper](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/spf13/viper?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Go Report Card](https://goreportcard.com/badge/github.com/spf13/viper?style=flat-square)](https://goreportcard.com/report/github.com/spf13/viper)
-![Go Version](https://img.shields.io/badge/go%20version-%3E=1.15-61CFDD.svg?style=flat-square)
+![Go Version](https://img.shields.io/badge/go%20version-%3E=1.16-61CFDD.svg?style=flat-square)
 [![PkgGoDev](https://pkg.go.dev/badge/mod/github.com/spf13/viper)](https://pkg.go.dev/mod/github.com/spf13/viper)
 
 **Go configuration with fangs!**