Update bigdecimal version used in benchmark from 3.0.0 to 3.1.1

[tompng]

GitHub actions macos-latest has been updated to macos-15, and gem install bigdecimal -v 3.0.0 began to fail.

GH-Actions failing log

Operating System
  macOS
  15.5

...

Run bundle install
  bundle install
  gem install bigdecimal -v 3.0.0

...
ERROR:  Error installing bigdecimal:
	ERROR: Failed to build gem native extension.
...
compiling bigdecimal.c
bigdecimal.c:249:5: error: 'maybe_unused' attribute cannot be applied to types
  249 |     ENTER(1);
      |     ^

This problem seems to be fixed in 6d510e4 (released as bigdecimal-3.1.0)
Use bigdecimal 3.1.1 which is bundled in ruby-3.1.0