Drop PHP7 support and refactor migration file

[norbybaru]

Changes

• Drop PHP7 support to enforce PHP8 upgrade as PHP7 has reached end-of-life support including security fixes.
• Update migration file to publish migration file with current timestamp instead of the hard-coded timestamp. This is better for auditing to know when the file was generated.
• Update the installation step to include the publish step for migration files for customization or applying default migration. this allows as well migration files to be version-controlled on the project.
• Drop support for Laravel 5,6,7 as those have reached end-of-life support.

PS. This should probably be a major release to start migrating coding standards to PHP8.

[stevebauman]

Hey @norbybaru, appreciate the PR!

I actually wanted to keep this package's great backwards compatibility until it's necessary to break it. I don't see why these changes are necessary, besides the improved migration naming?