Get Compiler Warnings for Apex API Versions 9.0 Through 19.0

Apex classes and triggers saved at API versions 9.0 through 19.0 will be retired in a future release, and the Apex compiler now warns when you compile or deploy code at those versions (in Setup, Salesforce CLI, VS Code extensions, Web Console, and Tooling API). Update affected classes and triggers to API version 20.0 or later; managed package code at these versions must be fixed by the package developer. This retirement is unrelated to the Salesforce Platform API retirement and does not affect Visualforce, Flow, or Process Builder versions.