feat!: Drop support for Rails 5, 5.1, 5.2

[CloCkWeRX]

https://endoflife.date/rails

While Rails 5 experienced significant usage, versions less than 6 went EOL some years ago. This reduces the maintenance surface; while still keeping a fairly wide window

Related:

• feat!: Remove Ruby 2.6 and 2.7 as EOL #1490

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