Update docs to encourage using connection string instead of instrumentation key

[babakks]

As of this update, the support for instrumentation keys-based usages will end on March 2025:

On 31 March 2025, technical support for instrumentation key–based global ingestion in the Application Insights feature of Azure Monitor will end. After that date, your Applications Insights resources will continue to receive data, but we'll no longer provide updates or customer support for instrumentation key–based global ingestion.

So, this repo docs should be updated to reflect this. As I checked, the users just need to provide their connection string in place of the instrumentation key (as the TelemetryReporter class constructor's only argument), and they're good to go.

[lramos15]

Updated the docs