chore: limit support of Go versions to 1.20 & 1.21 (#274)

* chore: Update to Go 1.21

[#185100152](https://www.pivotaltracker.com/story/show/185100152)

* chore: change minimum supported version

Since we want to convey that this version is compatible with go 1.20 & 1.21, it makes sense to specify go 1.20 in go.mod instead of go 1.21

.github/workflows/run-tests.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        version: [ '1.19', '1.20' ]
+        version: [ '1.20', '1.21' ]
     name: Go ${{ matrix.version }}
     outputs:
       pr_number: ${{ github.event.number }}

go.mod

@@ -1,6 +1,6 @@
 module github.com/pivotal-cf/brokerapi/v10
 
-go 1.19
+go 1.20
 
 require (
 	code.cloudfoundry.org/lager/v3 v3.0.2