Deprecate instrumentationsLoaded

[unflxw]

As sdk.start() now starts the OpenTelemetry SDK asynchronously, the Appsignal.client.instrumentationsLoaded property is no longer needed, and this PR changes it to always be Promise.resolve(). See also the corresponding OpenTelemetry issue for details.

To do

• Remove documentation page https://github.com/appsignal/appsignal-docs/pull/1311
• Add deprecation warning (instrumentationsLoaded can become a getter which prints a warning saying to remove this line, but continues to return Promise.resolve() anyway) Add deprecation warning to instrumentationsLoaded #888
• Eventually remove it in v4