golang: Switch default go1.14 -> go1.15

Go1.14 is end-of-life by today as it will no longer be updated after Go1.16 was
released in Feb 2021:

        https://golang.org/doc/devel/release.html#go1.14
        https://golang.org/doc/devel/release.html#go1.16

Go1.15 is current oldstable - i.e. it is still supported and mature.

component/golang/buildout.cfg

@@ -113,7 +113,7 @@ bin	= ${:directory}/bin
 depends = ${gowork.goinstall:recipe}
 
 # go version used for the workspace (possible to override in applications)
-golang  = ${golang1.14:location}
+golang  = ${golang1.15:location}
 
 # no special build flags by default
 buildflags =