Migrating to .NET 8

[david-staniec-octopus]

Background

Due to .NET 6 going EOL on 12th November 2024, this PR removes .NET 6 as a target framework across Tentacle projects, and upgrades build scripts to use .NET 8.

MQS Handover Discussion

• Make sure all .net 6 references have been replaced with .net8 inside the repo
• Make sure not installing .net6 or .net8 where we don't need to (in the build shell script)
• Review / update all teamcity configuration to ensure we are not calling .net6 test / test names anymore
• Test build in Teamcity on a new / cloned build chain configuration to ensure the changes work as expected prior to us merging this PR to main
• Investigate and resolve cross platform build issue - referencing octo cli on .net6
• Test branch build of Tentacle in Octopus Server
• Should we create a significant new version of tentacle for .net8?

How to review this PR

Quality ✔️

Pre-requisites

• I have read How we use GitHub Issues for help deciding when and where it's appropriate to make an issue.
• I have considered informing or consulting the right people, according to the ownership map.
• I have considered appropriate testing for my change.