app/appengine: remove Go 1.11 from the dashboard
app/appengine: remove Go 1.11 from the dashboard

Per release policy¹, we only support and test the two latest
major versions (currently Go 1.13 and Go 1.12).

Previously:

• CL 164218 removed Go 1.10.
• CL 133895 removed Go 1.9.
• CL 95555 removed Go 1.8.
• CL 77931 removed Go 1.7.
• CL 37466 removed Go 1.6.
• CL 31767 removed Go 1.5.
• CL 20843 removed Go 1.4.

¹ https://golang.org/doc/devel/release.html#policy

Fixes golang/go#34096

Change-Id: I8ac7fcb6527e86706bdb7f140e3f05d3262f2476
Reviewed-on: https://go-review.googlesource.com/c/build/+/193477
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build