Remove platforms from manifest (#203)

Motivation:

Adding or raising the deployment platforms in the package manifest is a
SemVer major breaking change as consumers must also add or raise their
deployment platforms. This is a known limitation of SwiftPM.

Unforunately this means that it's very difficult for non-leaf packages
to adopt packages which declare their platforms in the manifest. Doing
so puts the brakes on adoption and ecosystem growth. For 'core' packages
like this one availability constraints should be expressed on
declarations rather than in the manifest.

Modifications:

- Remove platforms from the package manifest
- Add availability annotations to types which require it

Result:

This package can be more widely adopted

Support adding services while `ServiceGroup` is running (#199)

somewhat straight-forward implementation draft of 
#185

choices were made, what do we think?

Remove deprecation of withGracefulShutdownHandler if compiler < 6 (#194)

Currently, if compiler >= 6 then there are 2 implementations. One is
deprecated and one is not.
But if compiler is < 6 then there is only one implementation, which is
deprecated. I think this was by mistake.

[CI] Adopt GitHub actions (#187)

# Motivation

We want to switch our CI to GitHub actions.

# Modification

This PR sets up the reusable workflows from NIO.

# Result

More modern CI.

Change 'unsafeDowncast' to 'as!' (#186)

Motivation:

The 'unsafeDowncast' can cause a miscompile leading to unexpected
runtime behaviour.

Modifications:

- Use 'as!' instead

Result:

No miscompiles on 5.10

add support for trapping sigabrt, sigill, sigsegv (#184)

- sigabrt: usually generated by the abort() function. Useful for cleanup prior to termination.
- sigill: issued if the user attempts to execute an illegal, malformed, or privileged instruction.
- sigsegv: issued if the user makes an invalid memory reference, such as dereferencing a null or invalid pointer.

Stop using renamed pattern which can cause compiler crash (#183)

Motivation:

Deprecation with rename fixit has been seen to cause compiler crashes
in 5.x compiler versions.

Modifications:

Deprecate with message for 5.x compilers.

Result:

Compilation without failure in all scenarios

Support compiling against Musl (#182)

`cancelOnGracefulShutdown` hangs, if cancellation is not immediately (#…

…177)

* Add test case

* Fix tests

* Make code simpler

* Fix Sendable

* swift-format

Conform `ServiceGroup` to `Service` (#172)