Update outdated Go toolchain version for .drone.yml

[typeless]

Other CI pipelines already use Go1.11. So, we update this one too.

[codecov-io]

Codecov Report

Merging #5146 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5146      +/-   ##
==========================================
- Coverage   37.48%   37.47%   -0.01%     
==========================================
  Files         309      309              
  Lines       45782    45782              
==========================================
- Hits        17162    17158       -4     
- Misses      26159    26160       +1     
- Partials     2461     2464       +3

Impacted Files	Coverage Δ
modules/process/manager.go	76.81% <0%> (-4.35%)	⬇️
models/repo_indexer.go	50.84% <0%> (-1.28%)	⬇️
models/repo_list.go	63.37% <0%> (+1.16%)	⬆️

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[lafriks]

Nooooo.... Please send PR to change it back as it was intentionally left this way to test if gitea builds with lowest supported golang toolchain version

[lunny]

@lafriks for Gitea v1.7 maybe we should change the lowest go version from go1.9 to go1.11 for using go modules?

[lafriks]

@lunny probably but this change should come also with that migration to go modules and updated docs

[lunny]

OK