Install both test runtimes in the deploy workflow

[MichaelGHSeg]

The 3.0.0 release failed in verify:

Testhost process for source(s) '.../Tests/bin/Debug/net6.0/Tests.dll' exited with error:
You must install or update .NET to run this application.
Framework: 'Microsoft.NETCore.App', version '6.0.0'
The following frameworks were found: 8.0.6, 8.0.22, 8.0.31, 9.0.6, 9.0.20, 10.0.8, 10.0.11, 10.0.12

Tests targets net10.0;net6.0. ci.yml installs both runtimes; deploy.yml still pinned 8.0.x in both of its jobs, so there was no 6.0 runtime to execute that leg. My oversight when scoping deploy.yml — I updated the SDK pin in ci.yml and not here.

Nothing was published: verify gates deploy, and NuGet is still on 2.6.0.

Re-tagging

deploy.yml runs from the tagged commit, so merging this does not change what 3.0.0 runs. The tag has to be recreated once this lands:

git push origin :refs/tags/3.0.0 && git tag -d 3.0.0
git checkout main && git pull
git tag 3.0.0 && git push origin 3.0.0

3.0.0 is unused on NuGet, so the number is free to reuse.