Update ruby-version, gemfile & README to use 3.1.2

[scheals]

Complete the following REQUIRED checkboxes:

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the location of change: brief description of change format, e.g. String spec: Update instructions for clarity

Complete the following checkboxes ONLY IF they are applicable to your PR. You can complete them later if they are not currently applicable:

• I have ensured any exercise files included in this PR have passed all of their tests

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1. Because:

The Ruby version curriculum is moving towards is 3.1.2 due the fact that only Ruby 3.1+ supports openSSL 3 and previous versions do not - making using 3.0.3 on Ubuntu 22.04 and such impossible without using workarounds including reverting to openSSL 1.1.

Part of TheOdinProject/curriculum#24112

2. This PR:

• Updates bundler to version shipped with 3.1.2
• Updates gems and ruby-version through bundle update
• Changes wording in README file so it says 3.1.2 instead of 3.0.3

3. Additional Information:

[scheals]

I have gone through someone's completed repo and made sure every tests pass without any issues.