[13.x] Fix number abbreviation rollover between unit tiers

[Button99]

This PR fixes Number::abbreviate() and Number::forHumans() when a value rounds up across a unit boundary.

Before the change, values like 999500 were formatted as 1,000K instead of 1M, the same thing affected higher tiers.

e.g.

Before:
  - `Number::abbreviate(999500)` → `1,000K`
  - `Number::forHumans(999500)` → `1,000 thousand`

  After:
  - `Number::abbreviate(999500)` → `1M`
  - `Number::forHumans(999500)` → `1 million`