golang: v↑ go1.15

Go1.15 is incremental improvement over Go1.14 with better compiler and runtime:

https://blog.golang.org/go1.15
https://golang.org/doc/go1.15

Don't drop support for Go1.13 yet, as that (no longer supported) Go
release is still being used by replication-manager and restic
components.

Remain default at Go1.14 yet.

Switch helloworld to Go1.15 and test this patch on that
software-release.

component/golang/buildout.cfg

@@ -59,6 +59,16 @@ md5sum = ae124560e0185a82883b57cc9662aa73
 environment-extra =
   GOROOT_BOOTSTRAP=${golang14:location}
 
+[golang1.15]
+<= golang-common
+url = https://golang.org/dl/go1.15.5.src.tar.gz
+md5sum = 12a60d319ff58c03c323e8dcc125ae81
+
+# go1.15 needs go1.4 to bootstrap
+environment-extra =
+  GOROOT_BOOTSTRAP=${golang14:location}
+
+
 # ---- infrastructure to build Go workspaces / projects ----
 
 # gowork is a top-level section representing workspace

component/helloweb/buildout.cfg

@@ -25,7 +25,7 @@ parts =
 install =
     lab.nexedi.com/nexedi/helloweb/go/...
 
-golang  = ${golang1.14:location}
+golang  = ${golang1.15:location}
 
 # -*- go -*-
 [helloweb-go]