Update bcrypt to 3.1.18

What does this MR do and why?

I originally thought we'd need this for Ruby 3 compat, but I don't think we actually do; this issue reports a compilation error but we are not experiencing this, and that issue is unresolved for the user anyway so it wouldn't be resolved by any recent release.

However, since I already made the change I thought I'd put this in an MR anyway since maybe we want to remain up to date anyway.

Changes:

3.1.18 May 16 2022
  - Unlock GVL when calculating hashes and salts [GH #260]
  - Fix compilation warnings in `ext/mri/bcrypt_ext.c` [GH #261]

3.1.17 Mar 14 2022
- Fix regex in validators to use \A and \z instead of ^ and $ [GH #121]
- Truncate secrets greater than 72 bytes in hash_secret [GH #255]
- Assorted test and doc improvements

https://github.com/bcrypt-ruby/bcrypt-ruby/blob/master/CHANGELOG

These sound all fairly minor except for https://github.com/bcrypt-ruby/bcrypt-ruby/pull/255, which I am not sure how it may affect existing data.

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Related to #369392 (closed)