rename top matrix workflow object 'swift'->'config'

[rnro]

Motivation:

The use of 'swift' here is very arbitrary and adds no value. The object is only there at all because GitHub Actions seems to require it.

Modifications:

Rename the top-level object from 'swift' to 'config' to maybe add value to the intent of the object and hopefully be less cryptic.

Result:

Everything should just keep working.

[rnro]

Example of this change working https://github.com/apple/swift-nio/actions/runs/12294108306/job/34308220995?pr=3024

 Inputs
    name: Unit tests
    matrix_string: {"config":[{"name":"5.9","image":"swift:5.9-jammy","swift_version":"5.9","platform":"Linux","command":"swift test","command_arguments":"-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error","setup_command":"","runner":"ubuntu-latest"},{"name":"5.10","image":"swift:5.10-jammy","swift_version":"5.10","platform":"Linux","command":"swift test","command_arguments":"-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error","setup_command":"","runner":"ubuntu-latest"},{"name":"6.0","image":"swift:6.0-jammy","swift_version":"6.0","platform":"Linux","command":"swift test","command_arguments":"-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error","setup_command":"","runner":"ubuntu-latest"},{"name":"nightly-6.0","image":"swiftlang/swift:nightly-6.0-jammy","swift_version":"nightly-6.0","platform":"Linux","command":"swift test","command_arguments":"-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error","setup_command":"","runner":"ubuntu-latest"},{"name":"nightly-main","image":"swiftlang/swift:nightly-main-jammy","swift_version":"nightly-main","platform":"Linux","command":"swift test","command_arguments":"--explicit-target-dependency-import-check error","setup_command":"","runner":"ubuntu-latest"}]}
Complete job name: Unit tests / Unit tests / Linux (5.9)