🐛Fix concurrency issue getting abbreviations

[angularsen]

Fixes #1303

Change back to ConcurrencyDictionary, there was a subtle write-on-read when loading the initial values that could throw in UnitAbbreviationsCache.GetAbbreviations():

if (!AbbreviationsMap.TryGetValue(key, out IReadOnlyList<string>? abbreviations))
    AbbreviationsMap[key] = abbreviations = ReadAbbreviationsFromResourceFile(unitInfo.QuantityName, unitInfo.PluralName, culture);

System.InvalidOperationException: Operations that change non-concurrent collections must have exclusive access. A concurrent update was performed on this collection and corrupted its state. The collection's state is no longer correct.
    at System.Collections.Generic.Dictionary`2.FindValue(TKey key)
    at System.Collections.Generic.Dictionary`2.TryGetValue(TKey key, TValue& value)
    at UnitsNet.UnitAbbreviationsCache.GetAbbreviations(UnitInfo unitInfo, IFormatProvider formatProvider)
    at UnitsNet.UnitAbbreviationsCache.TryGetUnitAbbreviations(Type unitType, Int32 unitValue, IFormatProvider formatProvider, String[]& abbreviations)
    at UnitsNet.UnitAbbreviationsCache.GetUnitAbbreviations(Type unitType, Int32 unitValue, IFormatProvider formatProvider)

[codecov]

Codecov Report

Patch coverage: 100% and no project coverage change.

Comparison is base (2dd76b8) 84% compared to head (f50a1b8) 84%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1305   +/-   ##
======================================
  Coverage      84%     84%           
======================================
  Files         336     336           
  Lines       31918   31920    +2     
======================================
+ Hits        26903   26905    +2     
  Misses       5015    5015           

Files Changed	Coverage Δ
UnitsNet/CustomCode/UnitAbbreviationsCache.cs	88% <100%> (+<1%)	⬆️

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