Improve code style by applying analyzer rules

[romanett]

Proposed changes

Improve code style by applying analyzer rules:

• pattern matching
• var
• expressions for properties
• simplify member access (namespaces)
• cleanup usings
• Coalese assignment (??=)
• fix analyzer warnings (Invariant Culture)

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.

Further comments

[codecov]

Codecov Report

Attention: Patch coverage is 59.96785% with 747 lines in your changes missing coverage. Please review.

Project coverage is 55.45%. Comparing base (14d78ca) to head (5fe0068).

Files with missing lines	Patch %	Lines
...aries/Opc.Ua.Server/NodeManager/CoreNodeManager.cs	5.00%	65 Missing and 11 partials ⚠️
Libraries/Opc.Ua.Client/Session/Session.cs	63.59%	64 Missing and 11 partials ⚠️
...ries/Opc.Ua.Client/Session/SessionClientBatched.cs	60.66%	51 Missing and 8 partials ⚠️
...ies/Opc.Ua.Server/Diagnostics/CustomNodeManager.cs	55.78%	33 Missing and 9 partials ⚠️
...pc.Ua.Gds.Server.Common/ApplicationsNodeManager.cs	61.90%	14 Missing and 10 partials ⚠️
...raries/Opc.Ua.Client/Subscription/MonitoredItem.cs	48.88%	23 Missing ⚠️
...raries/Opc.Ua.Configuration/ApplicationInstance.cs	66.66%	18 Missing and 5 partials ⚠️
Libraries/Opc.Ua.Server/Diagnostics/AuditEvents.cs	46.15%	19 Missing and 2 partials ⚠️
...ies/Opc.Ua.Server/NodeManager/MasterNodeManager.cs	64.70%	10 Missing and 8 partials ⚠️
...es/Opc.Ua.Client.ComplexTypes/ComplexTypeSystem.cs	10.52%	16 Missing and 1 partial ⚠️
... and 65 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2973      +/-   ##
==========================================
- Coverage   55.49%   55.45%   -0.05%     
==========================================
  Files         352      352              
  Lines       67853    67457     -396     
  Branches    13904    13899       -5     
==========================================
- Hits        37658    37411     -247     
+ Misses      26080    25931     -149     
  Partials     4115     4115              

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