Update the release.yml file with the latest label changes

…age mode CI (#44)

### Motivation

We're trying to lock in strict concurrency across the package ecosystem
and standardise on how that's tested.

### Modifications

- Add `StrictConcurrency=complete` feature flag to all targets in the
package manifest
- Remove now superfluous strict concurrency compiler flag in CI runs
- Add a new CI pipeline that tests the package with Swift 6 language
mode.

### Result

- Package configuration consistent with others adopting strict
concurrency.
- Swift 6 language mode pipeline established to catch regressions.

CI use 6.1 nightlies now that Swift development is happening in the 6.1
branch

Only apply standard swift settings on valid targets. The current check
ignores plugins but that is not comprehensive enough.

Rename nightly_6_1 params to nightly_next; see
apple/swift-nio#3122

Add static SDK CI workflow which runs on commits to PRs, merges to main
and daily on main.

Motivation:

Swift 6.1 has been released, we should add it to our CI coverage.

Modifications:

Add additional Swift 6.1 jobs where appropriate in main.yml,
pull_request.yml

Result:

Improved test coverage.

Motivation:

Swift 5.9 is no longer supported, we should bump the tools version and
remove it from our CI.

Modifications:

* Bump the Swift tools version to Swift 5.10
* Remove Swift 5.9 jobs where appropriate in main.yml, pull_request.yml

Result:

Code reflects our support window.