Fix #2259 open redirect vulnerability in echo.StaticDirectoryHandler (used by e.Static, e.StaticFs etc)

[aldas]

Fix #2259 open redirect vulnerability in echo.StaticDirectoryHandler (used by e.Static, e.StaticFs etc)

Similar fix as #1771 had. path.Clean() could be alternative but potentially causes too much changes to path.

remove pre Go1.16 and after differences not to duplicate stuff anymore

[codecov]

Codecov Report

Merging #2260 (3154abd) into master (d48197d) will decrease coverage by 0.04%.
The diff coverage is 84.95%.

@@            Coverage Diff             @@
##           master    #2260      +/-   ##
==========================================
- Coverage   92.40%   92.35%   -0.05%     
==========================================
  Files          37       37              
  Lines        4423     4436      +13     
==========================================
+ Hits         4087     4097      +10     
- Misses        245      247       +2     
- Partials       91       92       +1     

Impacted Files	Coverage Δ
context_fs.go	78.57% <76.47%> (ø)
echo_fs.go	84.94% <84.70%> (ø)
group_fs.go	100.00% <100.00%> (ø)
middleware/csrf.go	89.42% <0.00%> (+0.64%)	⬆️

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