Update target frameworks and other infrastructure changes

[Romfos]

changes:

• [Breaking Change] Update target frameworks: .NET 8, .NET Standard 2.0
• Drop EOL .NET 6/7 platforms from testing matrix
• Update github actions steps versions
• [Breaking Change] Remove legacy obsolete API
• [Breaking Change] Mark as obsolete api CompatArg with pre c# 7.0 support

This PR is a first part of preparation to version 6.0.0 #830

please, join to discussion

[dtchepak]

Looks great @Romfos , thanks! I've left a few comments/questions but probably ok to merge as-is.

[dtchepak]

I will merge this once v5.3 PR is merged and released. 👍

[dtchepak]

@Romfos Thanks for all your contributions to NSub. Would you be open to joining the contributors team?

No obligations in terms of amount of involvement or anything. It can be as little or as much as you like.

You are welcome to email me@davesquared.net if you have any questions. 🙇

[Romfos]

@Romfos Thanks for all your contributions to NSub. Would you be open to joining the contributors team?

No obligations in terms of amount of involvement or anything. It can be as little or as much as you like.

You are welcome to email me@davesquared.net if you have any questions. 🙇

Yea, as I see - I already have Contributor status

[dtchepak]

@Romfos Thanks for all your contributions to NSub. Would you be open to joining the contributors team?
No obligations in terms of amount of involvement or anything. It can be as little or as much as you like.
You are welcome to email me@davesquared.net if you have any questions. 🙇

Yea, as I see - I already have Contributor status

Thanks @Romfos ! Invite sent :)