Drop .NET Core 2.1 to fix Azure dependency security issue

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

See #284 (Dependabot)

We have a security issue in the Azure Dependency and are unable to upgrade to mitigate the issue without removing support for .NET Core 2.. Since that target framework is long obsolete I've chosen to do this without a major update. The next version will be v2.1.0

[codecov]

Codecov Report

Merging #286 (67f2a67) into main (108a615) will increase coverage by 0%.
The diff coverage is n/a.

@@         Coverage Diff         @@
##           main   #286   +/-   ##
===================================
  Coverage    85%    85%           
===================================
  Files        76     76           
  Lines      2169   2165    -4     
  Branches    303    303           
===================================
- Hits       1855   1853    -2     
+ Misses      229    227    -2     
  Partials     85     85           

Flag	Coverage Δ
unittests	85% <ø> (+<1%)	⬆️

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

Impacted Files	Coverage Δ
.../ImageSharp.Web/Caching/PhysicalFileSystemCache.cs	98% <ø> (-1%)	⬇️
...arp.Web/ImageSharpRequestAuthorizationUtilities.cs	90% <ø> (ø)
.../ImageSharp.Web/Middleware/ImageSharpMiddleware.cs	80% <ø> (ø)
...eSharp.Web/Providers/PhysicalFileSystemProvider.cs	100% <ø> (ø)
...c/ImageSharp.Web/Providers/WebRootImageProvider.cs	0% <ø> (ø)
.../Synchronization/RefCountedConcurrentDictionary.cs	76% <ø> (+4%)	⬆️

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