Rewrite KeyVault access logic to recommended lib

[SandGrainOne]

Description

• Upgrade AccessToken project to .net v6.0
• Change to recommended library for KeyVault interaction
• Rename ISigningKeysResolver and SigningKeysResolver to IPublicSigningKeyProvider and PublicSigningKeyProvider respectively
• Clean up in code and comments here and there.

Related Issue(s)

• 2

• 25

Verification

• [x] Your code builds clean without any errors or warnings
• [x] All tests run green
• [ ] Manual testing done (required)
• [ ] Relevant automated test added (if you find this hard, leave it and we'll help out)

[SandGrainOne]

I excluded PublicSigningKeyProvider from code coverage as it is mostly about KeyVault access. It's possible to improve coverage a little bit by moving the caching to a separate class, but I'm lazy today.

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

83.3% Coverage
0.0% Duplication