Remove the integration tests workflow from the main and scheduled jobs.
### Motivation:
There are no integration tests in this repository.
(see
https://github.com/apple/swift-nio-transport-services/actions/runs/11833082880)
### Modifications:
Remove the integration tests workflow from the main and scheduled jobs.
### Result:
No more scheduled run failures.
### Motivation:
* `main.yml` and `scheduled.yml` are mostly duplicative.
* Scheduled runs failed because of a deprecated reference to a Swift 5.8
pipeline
### Modifications:
* Unify `main.yml` and `scheduled.yml`
* Remove the reference to the 5.8 pipeline
### Result:
Working scheduled runs.
### Motivation:
To migrate to GitHub actions and centralised infrastructure.
### Modifications:
Changes of note:
* Adopt swift-format using rules from SwiftNIO. Modified so that it
ignores `NoAssignmentInExpressions` in some `XCTAssert` functions.
* Remove scripts and docker files which are no longer needed
### Result:
Feature parity with old CI.
Rick Newton-Rogers