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.