fix: move to go1.15 (#10660)

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.14-alpine as builder
+FROM golang:1.15-alpine as builder
 
 LABEL maintainer="MinIO Inc <dev@min.io>"
 

README.md

@@ -88,7 +88,7 @@ service minio start
 ```
 
 ## Install from Source
-Source installation is only intended for developers and advanced users. If you do not have a working Golang environment, please follow [How to install Golang](https://golang.org/doc/install). Minimum version required is [go1.14](https://golang.org/dl/#stable)
+Source installation is only intended for developers and advanced users. If you do not have a working Golang environment, please follow [How to install Golang](https://golang.org/doc/install). Minimum version required is [go1.15](https://golang.org/dl/#stable)
 
 ```sh
 GO111MODULE=on go get github.com/minio/minio

mint/preinstall.sh

@@ -38,7 +38,7 @@ $APT update
 $APT install gnupg ca-certificates
 
 # download and install golang
-GO_VERSION="1.14.7"
+GO_VERSION="1.15.2"
 GO_INSTALL_PATH="/usr/local"
 download_url="https://storage.googleapis.com/golang/go${GO_VERSION}.linux-amd64.tar.gz"
 if ! $WGET --output-document=- "$download_url" | tar -C "${GO_INSTALL_PATH}" -zxf -; then