bump minimum go version to 1.15

The oldest distributions that we now support are RHEL 8.4 and Fedora 33.
They both support go 1.15, let's bump.

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

.github/workflows/tests.yml

@@ -16,10 +16,10 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
 
-      - name: Set up Go 1.14
+      - name: Set up Go 1.15
         uses: actions/setup-go@v1
         with:
-          go-version: 1.14
+          go-version: 1.15
         id: go
 
       - name: Check out code into the Go module directory

README.md

@@ -51,7 +51,7 @@ The requirements for this project are:
 
 At build-time, the following software is required:
 
- * `go >= 1.14`
+ * `go >= 1.15`
  * `python-docutils >= 0.13`
 
 ### Build

go.mod

@@ -1,6 +1,6 @@
 module github.com/osbuild/osbuild-composer
 
-go 1.14
+go 1.15
 
 require (
 	cloud.google.com/go v0.75.0

tools/prepare-source.sh

@@ -2,7 +2,7 @@
 
 set -eux
 
-GO_VERSION=1.14.14
+GO_VERSION=1.15.13
 GO_BINARY=$(go env GOPATH)/bin/go$GO_VERSION
 
 # this is the official way to get a different version of golang