Chore | Upgrade IdentityModel dependencies

[lcheunglci]

Relates to #1601, the dependencies for the IdentityClient, IdentityModel JWT and IDOC are updated to a newer stable version i.e. from 6.8.0 to 6.18.0. I did notice when I checked again the latest version is now 6.19.0. Not sure, if I should upgrade to that one instead.

[AraHaan]

Since 6.19.0 is the latest stable why not test 18.0 and then later test 19.0 to see which one works better, if nothing changes I guess then do 19.0.

[codecov]

Codecov Report

Merging #1646 (7c620a1) into main (4e3aa5e) will decrease coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1646      +/-   ##
==========================================
- Coverage   71.54%   71.37%   -0.17%     
==========================================
  Files         291      291              
  Lines       61241    61241              
==========================================
- Hits        43817    43713     -104     
- Misses      17424    17528     +104     

Flag	Coverage Δ
addons	92.38% <ø> (ø)
netcore	74.91% <ø> (-0.18%)	⬇️
netfx	69.25% <ø> (-0.10%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...src/Microsoft/Data/SqlClient/SqlMetadataFactory.cs	81.66% <0.00%> (-10.84%)	⬇️
...ata/SqlClient/SqlConnectionTimeoutErrorInternal.cs	41.96% <0.00%> (-9.83%)	⬇️
...ActiveDirectoryAuthenticationTimeoutRetryHelper.cs	56.81% <0.00%> (-6.82%)	⬇️
.../src/Microsoft/Data/SqlClient/SNI/SNIMarsHandle.cs	87.89% <0.00%> (-4.49%)	⬇️
...nt/src/Microsoft/Data/ProviderBase/TimeoutTimer.cs	71.42% <0.00%> (-3.58%)	⬇️
...crosoft/Data/ProviderBase/DbReferenceCollection.cs	92.63% <0.00%> (-3.16%)	⬇️
...e/src/Microsoft/Data/SqlClient/SNI/SNITcpHandle.cs	60.62% <0.00%> (-2.34%)	⬇️
...Client/Reliability/Common/SqlRetryLogicProvider.cs	89.13% <0.00%> (-2.18%)	⬇️
...c/Microsoft/Data/SqlClient/SqlConnectionFactory.cs	68.85% <0.00%> (-0.82%)	⬇️
...rc/Microsoft/Data/ProviderBase/DbConnectionPool.cs	85.00% <0.00%> (-0.67%)	⬇️
... and 10 more

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[AraHaan]

Going to check 6.19.0 soon before settling with 6.18.0?

[lcheunglci]

Looks like there's another update since the last time I opened this PR for Identity.Client from 4.44.0 to 4.45.0 and IdentityModel.JsonWebToken and OpenIdConnect from 6.18.0 to 6.21.0, I'll update my PR to reflect that.

[AraHaan]

Hopefully Dependabot could be enabled for these too in the future.