Remove environmentVariables and apify secrets in the next major version #1579

The last step of the deprecation. It waits for usage to drop.

Scope

• Remove reading environmentVariables in apify run, @secret resolution (src/lib/secrets.ts), the apify secrets commands, and --allow-missing-secrets.
• When actor.json still has the key, fail with one line that points to apify env migrate.

Blocked until

• Telemetry shows environmentVariables in under 10% of pushes, the PRD target.
• The next major CLI version is planned.

Depends on

• The CLI deprecation (Deprecate environmentVariables, @secret references, and apify secrets #1577)
• Push telemetry (Record environmentVariables usage in apify push telemetry #1567)