.NET 6 migration - removal of implicit usings

[petrsvihlik]

Motivation

Which issue does this fix? Fixes #issue number

If no issue exists, what is the fix or new feature? Were there any reasons to fix/implement things that are not obvious?

Checklist

• [ ] Code follows coding conventions held in this repo
• [ ] Automated tests have been added
• [ ] Tests are passing
• [ ] Docs have been updated (if applicable)
• [ ] Temporary settings (e.g. variables used during development and testing) have been reverted to defaults
• [ ] Adheres to .NET Design Guidelines

How to test

What steps do you suggest to take to verify the correctness of the added/updated functionality?

[codecov[bot]]

Codecov Report

Merging #144 (d044b77) into master (5a1c78a) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #144   +/-   ##
=======================================
  Coverage   46.66%   46.66%           
=======================================
  Files          16       16           
  Lines         390      390           
  Branches       34       34           
=======================================
  Hits          182      182           
  Misses        205      205           
  Partials        3        3           

Impacted Files	Coverage Δ
WopiHost.Discovery/AsyncExpiringLazy{T}.cs	52.63% <ø> (ø)
WopiHost.Discovery/DiscoveryException.cs	0.00% <ø> (ø)
WopiHost.Discovery/DiscoveryOptions.cs	100.00% <ø> (ø)
...iHost.Discovery/FileSystemDiscoveryFileProvider.cs	100.00% <ø> (ø)
WopiHost.Discovery/HttpDiscoveryFileProvider.cs	0.00% <ø> (ø)
WopiHost.Discovery/TemporaryValue{T}.cs	100.00% <ø> (ø)
WopiHost.Discovery/WopiDiscoverer.cs	100.00% <ø> (ø)
WopiHost.FileSystemProvider/WopiFile.cs	0.00% <ø> (ø)
...iHost.FileSystemProvider/WopiFileSystemProvider.cs	0.00% <ø> (ø)
WopiHost.FileSystemProvider/WopiFolder.cs	0.00% <ø> (ø)
... and 4 more

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