Clean on go1.20

[aldas]

Go 1.20 "silently" changed how filepath.Clean works on Windows. This is not backported to 1.19.6 etc. Now we need todo some extra hoops to get static mw things work as they used to and securely

// As of 1.20 on Windows filepath.Clean has different behaviour on OS related filesystems so we need to use path.Clean
// which is more suitable for path coming from web but this has some caveats on Windows. When we eventually end up in
// os related filesystem Open methods we are getting different errors as earlier versions. As of 1.20 path checks are
// more strict on path you provide and consider path with [UNC](https://en.wikipedia.org/wiki/Path_(computing)#UNC)
// but missing host etc parts as invalid. Previously it would result you `fs.ErrNotExist`.
//
// So for 1.20@Windows we need to consider it as same not exist so we can continue next middleware/handler and not error
// which would result status 500 instead of potential route hit or 404.

[aldas]

@lammel please review

[codecov]

Codecov Report

Base: 92.80% // Head: 92.84% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (558bf90) compared to base (b888a30).
Patch coverage: 63.63% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2406      +/-   ##
==========================================
+ Coverage   92.80%   92.84%   +0.03%     
==========================================
  Files          38       39       +1     
  Lines        4506     4503       -3     
==========================================
- Hits         4182     4181       -1     
+ Misses        236      234       -2     
  Partials       88       88              

Impacted Files	Coverage Δ
middleware/static.go	69.23% <55.55%> (+0.42%)	⬆️
middleware/static_other.go	100.00% <100.00%> (ø)
echo.go	95.47% <0.00%> (ø)

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[lammel]

LGTM