Release notes for 3.4.0 are pointing to the main branch and some links are broken

[fforjan]

Some of links into the release note from https://github.com/microsoft/testfx/releases/tag/v3.4.0 like

And

Are pointing to the main branch which may be subject to change .

note that the links are already broken, for example for the second screenshot:

[Evangelink]

Hopefully I have now fixed the various references and ensured the samples in branch rel/3.4 are up-to-date.