Fix encoding detection on PHP 8.1

[come-nc]

Also get rid of deprecated utf8_encode

Spin-off of sabre-io/http#182

[codecov]

Codecov Report

Merging #1404 (68a1309) into master (130abb7) will decrease coverage by 0.12%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1404      +/-   ##
============================================
- Coverage     97.19%   97.07%   -0.13%     
+ Complexity     2826     2820       -6     
============================================
  Files           175      175              
  Lines          8352     7699     -653     
============================================
- Hits           8118     7474     -644     
+ Misses          234      225       -9     

Impacted Files	Coverage Δ
lib/DAV/StringUtil.php	100.00% <100.00%> (ø)
lib/DAVACL/PrincipalCollection.php	28.57% <0.00%> (-11.43%)	⬇️
lib/DAV/Auth/Backend/IMAP.php	22.22% <0.00%> (-4.10%)	⬇️
lib/DAV/PropertyStorage/Plugin.php	91.17% <0.00%> (-1.69%)	⬇️
lib/DAV/Xml/Request/ShareResource.php	90.00% <0.00%> (-1.67%)	⬇️
lib/DAV/Auth/Backend/File.php	92.30% <0.00%> (-1.03%)	⬇️
lib/CalDAV/SharedCalendar.php	95.91% <0.00%> (-1.01%)	⬇️
lib/DAV/Browser/MapGetToPropFind.php	90.90% <0.00%> (-0.76%)	⬇️
lib/DAV/Xml/Property/SupportedReportSet.php	95.83% <0.00%> (-0.72%)	⬇️
lib/DAVACL/FS/Collection.php	88.23% <0.00%> (-0.66%)	⬇️
... and 132 more

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[mbiebl]

Is a new sabre/dav release planned soonish with this fix? As mentioned in #1405, I've run into this issue with vCards containing names with Slovenian characters.