Update Buildvana.Sdk for saner CI

[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 Buildvana.Sdk 1.0.0-alpha.7 -> 1.0.0-alpha.9
• Remove setup of an additional .NET Core version in CI workflow, as the build tools (namely ExportAnnotation) no longer need it.

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

[codecov-io]

Codecov Report

Merging #14 into main will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #14   +/-   ##
=======================================
  Coverage   66.66%   66.66%           
=======================================
  Files           3        3           
  Lines         105      105           
  Branches       21       21           
=======================================
  Hits           70       70           
  Misses         25       25           
  Partials       10       10           

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[rdeago]

Let's recap:

• builds still fail with .NET 5.0-rc1, apparently [by no fault of our own[(https://github.com/Update dependencies improve templates, fix CI workflow, support net46 #11#issuecomment-704071805)
• the Buildvana.Sdk update made Ubuntu builds succeed at least with .NET Core 3.1
• LGTM can build the solution at last! The problem was that by default LGTM does a nuget restore before building, then builds with MSBuild 15.0. Their versions of nuget and MSBuild don't support centrally-managed package versions, so they were using terribly old (and incompatible) versions of dependencies (thanks NuGet for defaulting to the oldest version of everything!)
• actions/labeler fails. The error comes from the Minimatch constructor, that apparently is passed an object instead of an array of strings. Pulling actions/labeler from the main branch instead of the 2.2.0 tag would somewhat help in debugging the issue, as it now uses JSON.stringify instead of toString in the debug message that shows the globs being evaluated. Since after changing the labeler version in the workflow file GitHub seems to be still using 2.2.0, there's no choice but to merge this PR and open a new one to try the updated workflow.