Allow setting copyright year

[IvanRibakov]

🛠️ Description

Make use of the copyright_year configuration field and allow overriding it at runtime via command flag. This way users that want the old behaviour can simply ensure that they are not setting the copyright_year in the config file.

For the rest of the users this enables a variety of new usage patterns, for example:

• Use copyright_year in the configuration file to capture the project creation date (the one used during the initial invocation of copywrite headers)
• Use -y/--copyright-year flag in various dev automation tools to stamp new source files with the year they were first created.

🔗 External Links

N/A

👍 Definition of Done

• New functionality works?
• Tests added?

🤔 Can be merged upon approval?

✅

[hashicorp-cla-app]

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