Update 1 projects in ImageResizer.Plugins.DiskCache.WebTest.sln to LTS (or current) version

[github-actions[bot]]

There are 1 projects in ImageResizer.Plugins.DiskCache.WebTest.sln that target a .NET version which is no longer supported. This is an auto-generated issue, detailed and discussed in dotnet/docs#22271.

TFM in project	Target version	End of life	Release notes	Nearest LTS TFM version
ImageResizer.Plugins.DiskCache.WebTest.csproj	v4.5	January, 12 2016	v4.5 release notes	net48

Consider upgrading to either the current release, or the nearest LTS TFM version.

If any of these projects are intentionally targeting an unsupported version, you can optionally configure to ignore this automated issue. Create a file at the root of the repository, named dotnet-versionsweeper.json and add an ignore entry following the globbing patterns detailed here.

{
    "ignore": [
        "**/tests/DiskCacheWebTest/ImageResizer.Plugins.DiskCache.WebTest.csproj"
    ]
}