Historically brokerapi has required use of the
[`lager`](https://github.com/cloudfoundry/lager) logger. In Go 1.21,
structured logging was introduced into the Go standard library via the
[`log/slog`](https://pkg.go.dev/log/slog) package, and `slog`
[compatability was added](cloudfoundry/lager@4bf4955)
to `lager`.

`brokerapi` has been modified to require a `slog` logger to be passed
rather than a `lager` logger. This allows users a choice of logger.
Users who still want to use lager can easily do that using the
lager/slog compatability:
```go
logger := lager.NewLogger(name)
brokerAPI := brokerapi.New(serviceBroker, slog.New(lager.NewHandler(logger)), credentials)
```

And users who want to use `slog` or an `slog`-compatible logger can do
that instead.

A key advantage is that `lager` is no longer a dependency of this
package, which simplifies package management for apps that use brokerapi
and other libraries which use `lager`.

Externally we take a *slog.Logger, but internally we wrap this
almost everywhere

Previously some of the public API had been altered to take the
(internal) Blog logger. This is not correct, so the public API should
now consistely take a *slog.Logger, and a Blog logger now implements the
slog.Handler interface so can easily be converted to a slog.Logger.