remove deprecations related with rounding mode

[yukideluxe]

Hello @RubyMoney/core and contributors!

Another deprecation out of the way before releasing a new major version! 🎉 This one was originally introduced in this PR.

As explained in this comment, the rationale for changing the default rounding mode is:

the default rounding mode is ROUND_HALF_EVEN and they way it works might not be expected by people using this gem. It works best when aggregating with bigger amount of data and has an effect of minimizing rounding error, however when dealing with one off cases (for example rounding the result of a currency exchange) it's probably not as useful.

Out of curiosity – and to make sure we’re making the right call! – I asked ChatGPT for a summary of the different rounding modes. Besides summarizing them, it also brought up two interesting points:

• As of Ruby 3.2, the default rounding mode is now BigDecimal::ROUND_HALF_UP. I am always down on using the defaults. I double-checked it in the official docs:
  

• It also noted: "Use ROUND_HALF_EVEN when doing financial calculations to reduce cumulative rounding error". This had me thinking because, after all, this gem is called money. But as @antstorm pointed out, many users might not be doing large-scale aggregations and could get unexpected results with ROUND_HALF_EVEN because of the way it behaves (more about that at the bottom).

I’m inclined to go ahead and change the default to ROUND_HALF_UP, especially since it's easy to override if someone needs the previous behavior. Plus, this comment mentions that Stripe uses ROUND_HALF_UP so I am convinced 💪🏻

PS: Quick ChatGPT summary of the difference between the two modes and a table with examples:

• With ROUND_HALF_UP, every .5 always rounds up. This can introduce a small upward bias over time.
• With ROUND_HALF_EVEN, .5 values sometimes round up, sometimes down — helping cancel out rounding bias in large datasets. This is ideal for financial, banking, and accounting software.

[mrsimo]

Do we want to update something more in the README? In the example code, for example, it passes BigDecimal::ROUND_HALF_UP to the round method, even though it's the new default.

[yukideluxe]

good catch! done! dc3f52b

[sunny]

🙌🏻