[Deprecation] `Facter::Core::Execution.exec`

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Summary

Facter::Core::Execution.exec is explicitly deprecated in favor of Facter::Core::Execution.execute.

Evidence

• lib/facter/custom_facts/core/execution.rb:97 marks exec as deprecated with replacement guidance (Use #{execute} instead).
• lib/facter/custom_facts/core/execution.rb:100 delegates exec to @@impl.execute(command, on_fail: nil).
• spec/custom_facts/core/execution_spec.rb:39 verifies #exec delegation behavior.

Proposed Change

• Remove Facter::Core::Execution.exec.
• Keep execute as the public command execution API.

Compatibility / Risk

• Medium risk for custom facts/plugins calling Facter::Core::Execution.exec.
• Migration is direct and low-complexity (exec(cmd) -> execute(cmd, on_fail: nil) as needed).

Implementation Notes

• Update internal call sites first.
• Update public API docs/examples to only mention execute.
• If needed, include a clear release-note migration snippet.

Acceptance Criteria

• Facter::Core::Execution.exec no longer exists.
• Internal code uses execute only.
• Test suite no longer expects exec delegation.

Suggested Tests

• Unit tests for execute success/failure behavior (on_fail, timeout, logger).
• Negative API test for removed exec entry point if compatibility policy requires explicit error.

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Marked deprecated in documentation in 757b458.

There appears to be no explicit deprecation warning output for this. We should add that before removing the method.