Add BigDecimal to gemspec for Ruby 3.3/4

[womblep]

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Ruby 3.3 displays a warning when using pg-1.5.6

/usr/local/bundle/gems/pg-1.5.6/lib/pg/basic_type_registry.rb:229: warning: bigdecimal was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.4.0. Add bigdecimal to your Gemfile or gemspec. Also contact author of pg-1.5.6 to add bigdecimal into its gemspec.

[larskanis]

This is already fixed by #556

[womblep]

@larskanis great! I couldnt find an issue mentioning BigDecimal, I didnt look in PRs. Any idea when the next Gem will come out with this fix?