deprecate building for mips

[techknowlogick]

as title

[codecov-io]

Codecov Report

Merging #14174 (6a26dca) into master (3175d08) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #14174      +/-   ##
==========================================
- Coverage   42.01%   42.01%   -0.01%     
==========================================
  Files         733      733              
  Lines       78704    78704              
==========================================
- Hits        33070    33065       -5     
- Misses      40202    40205       +3     
- Partials     5432     5434       +2     

Impacted Files	Coverage Δ
modules/git/repo_base_nogogit.go	63.63% <0.00%> (-9.10%)	⬇️
modules/indexer/stats/db.go	48.00% <0.00%> (-8.00%)	⬇️
modules/git/utils.go	77.77% <0.00%> (-2.78%)	⬇️
services/pull/pull.go	42.35% <0.00%> (-0.51%)	⬇️
modules/process/manager.go	75.00% <0.00%> (+2.50%)	⬆️

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[zeripath]

how comes?

[techknowlogick]

@zeripath a grep of our logs from dl.gitea.io shows that it isn't being downloaded, and the downloads from github (https://somsubhra.com/github-release-stats/?username=go-gitea&repository=gitea) show that the number minimal, could even be bots.

It'll also reduce our release step build time, as it would be 3 less cross-compiles needed

[lunny]

We may also mentioned this on docs.