[APPSEC-60654] Emit warning when datasource app_sec_spans is used

[TanguyLichau]

What does this PR do?

As part of the appsecspan to trace track migration, we need to update the terraform provider to stop people from using the app_sec_spans datasource which converts to the appsecspan track in security-monitoring-api.
No breaking changes are introduced, only a warning message is displayed

Summary of the changes :

• Display warning message when datasource is app_sec_spans datasource
• Adds test for message

Warning message:

app_sec_spans datasource is deprecated
Use data_source = "spans" and add @appsec.security_activity:* to your query to keep the same behavior