[Server] [Regression] fix Sampling Group disposing m_shutdownEvent in Shutdown Method. Fix double call of Shutdown Method

[romanett]

• fix Sampling Group disposing m_shutdownEvent in Shutdown Method.
• Dont dispose running task, will be cleaned up by GC once shutdown event is reached
• fix double call of Shutdown Method
• regression from [Server] Fix Sampling Group creating a new long running task for every Client AddMonitoredItems Method Call #2984

Related Issues

• Fixes SamplingGroup throws ObjectDisposedException if all MonitoredItems are removed from a Subscription #3065

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 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 56.98%. Comparing base (d5fd96b) to head (4b7adee).
Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
...braries/Opc.Ua.Server/NodeManager/SamplingGroup.cs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3070      +/-   ##
==========================================
- Coverage   57.02%   56.98%   -0.04%     
==========================================
  Files         357      357              
  Lines       68951    68981      +30     
  Branches    14156    14161       +5     
==========================================
- Hits        39317    39312       -5     
- Misses      25395    25424      +29     
- Partials     4239     4245       +6     

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