Deprecate threadsafe error

[connorszczepaniak-wk]

This package is no longer particularly useful in modern Go. A vast majority of use cases I've seen look like this:

wg := &sync.WaitGroup{}
wg.Add(1)
e := terr.New()

go func() {
    defer wg.Done()
    err := doSomething()
    if err != nil {
        e.Set(err)
        return
    }
    // ...
}()

wg.Wait()
if e.Get() != nil {
    return e.Get()
}

Compare this to using an errgroup instead:

eg := &errgroup.Group{}

eg.Go(func() error {
    err := doSomething()
    if err != nil {
        return err
    }
    // ...
}()

err := eg.Wait()
if err != nil {
    return err
}

Far more concise and more idiomatic.

For cases that don't match this, atomic.Pointer[error] is a built-in threadsafe error that requires no additional implementation.

[simeonsteward-wk]

+10 adding deprecations needs no testing

[simeonsteward-wk]

@Workiva/release-management-p

[rmconsole-wf]

+1 from RM

[hancohaj-spec]

9fc51ad