Upgrade of Microsoft.Extensions.Caching.Memory dependency broke .NET Core 2.1 (LTS) support

[larsw]

Hi,

With reference to #37, #38 and #39:

By bumping the dependency from 2.0.2 to 2.2.0, the netstandard2.0 target can't be used with projects targeting .NET Core 2.1 (LTS) without a bump to the now unsupported .NET Core 2.2 or up to .NET Core 3.1 (LTS).

As far as I can tell, it would have been enough to bump the reference to 2.1.2 to ensure that the security vulnerability had been addressed, while keeping compatibility with the still-supported LTS version of .NET Core 2.x.