Update dependencies improve templates, fix CI workflow, support net46

[rdeago]

Types of changes

• Bugfix
• Enhancement (new and/or improved behavior)
• Refactoring (no functional changes)
• Code style update (no functional changes)
• Dependencies added, updated, or removed
• Build related changes
• CI related changes
• Documentation update
• Other (please describe below)

Proposed changes / fixed issues

• Update Mono.Cecil 0.11.2 -> 0.11.3
• Update Buildvana.Sdk 1.0.0-alpha.4 ->1.0.0-alpha.7
• Improve issue / PR templates.
• Implement build matrix in CI workflow.
• Identify action versions better.
• Standardize changelog style.
• Standardize copyright holders between license file and headers.
• Update various editor / analyzer / ReSharper settings.

Checklist

• My contribution is either my original work, or comes from projects with an MIT-compatible license, in which case I have updated the THIRD-PARTY-NOTICES file accordingly
• I have added and/or updated related documentation (if applicable)
• I have updated the "Unreleased changes" section in CHANGELOG.md according to the modifications I made

Other information

[rdeago]

Let's recap:

1. We have to install .NET Core 2.1 just to run ExportAnnotations from ReSharper.ExportAnnotations.Task. This is bad, but not an issue with our workflow. I'm opening an issue about this on the other repo.
2. Testing fails with .NET 5-rc1. Probably not an issue with out workflow either, as there are open issues about very similar problems: dotnet test error MSB4181: The "Microsoft.TestPlatform.Build.Tasks.VSTestTask" task returned false but did not log an error dotnet/sdk#13431 Strange message if test fails with preview 5.0.100-preview.2.20176.6 microsoft/vstest#2384
3. LGTM fails too! Although it was bound to fail anyway, due to the stray NuGet.config file currently present in the main branch and preventing restore to succeed, the build log for the PR shows a weird "not found" exception that would better investigated upon when the ExportAnnotation tool is fixed.

Keeping this PR on hold for now.

[rdeago]

To stop the vicious cycle formed by of this PR,Tenacom/ReSharper.ExportAnnotations#20, and #12, I'm merging this. We'll investigate CI /LGTM issues later.