Updates to 'Learning COBOL': add sources for facts and update some other links

[tobias-heuser]

Proposed changes

Add sources to the facts in How is COBOL being used today? and remove two mind-boggling facts I couldn't find a reliable source for, add a source for the comment in a business journal. Update links in Professional manuals and Learn more about recent COBOL advancements to reflect release of Enterprise COBOL for z/OS V6.5.

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• Bug fix (change which fixes one or more issues)
• New feature (change which adds functionality or features to the course)
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• This change requires an update to the course's z/OS environment

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