Use short for of substr to not need strlen()

[MorrisJobke]

Found by PHP inspections and makes the code more clear and also a bit more performant.

[codecov]

Codecov Report

Merging #8053 into master will not change coverage.
The diff coverage is 25%.

@@           Coverage Diff            @@
##             master   #8053   +/-   ##
========================================
  Coverage      51.7%   51.7%           
  Complexity    25457   25457           
========================================
  Files          1599    1599           
  Lines         95314   95314           
  Branches       1376    1376           
========================================
  Hits          49283   49283           
  Misses        46031   46031

Impacted Files	Coverage Δ	Complexity Δ
lib/private/DB/Adapter.php	14.28% <0%> (ø)	9 <0> (ø)	⬇️
apps/files_versions/lib/Storage.php	65.97% <0%> (ø)	103 <0> (ø)	⬇️
lib/private/legacy/template.php	30% <0%> (ø)	40 <0> (ø)	⬇️
lib/private/DB/AdapterSqlite.php	85.71% <100%> (ø)	7 <0> (ø)	⬇️
apps/files_trashbin/lib/Trashbin.php	72.28% <0%> (-0.25%)	136% <0%> (ø)
apps/files_trashbin/lib/Expiration.php	91.93% <0%> (+1.61%)	29% <0%> (ø)	⬇️