Require Swift 6

[aryan-25]

Motivation:

• This PR paves the way to land the new API discussed in v2 API Proposal Document #339.
• AWS Lambda Runtime v2 will require Swift 6 as it will enforce full Sendability checking.
• For us to land the PRs that work towards our new API, we need to require Swift 6.

Modifications:

• Change swift-tools-version from 5.9 to 6.0 in Package.swift.
• Use Swift 5 Language Mode for all targets.
• Modify Plugin.swift to compile error-free with Swift 6.

Result:

First step towards adopting Swift 6.

[fabianfett]

@swift-server-bot add to whitelist

[sebsto]

LGTM

[fabianfett]

Let's land #340 and then rebase on top of it.

[fabianfett]

LGTM, Let's get CI in order

[fabianfett]

API breakage and doc check fail because of 5.10 requirement. Merging to unblock further work for now. Will enforce at a later point in time again.