[ruby/json] Reduce encoding benchmark size

Profiling revealed that we were spending lots of time growing the buffer.
Buffer operations is definitely something we want to optimize, but for
this specific benchmark what we're interested in is UTF-8 scanning performance.

Each iteration of the two scaning benchmark were producing 20MB of JSON,
now they only produce 5MB.

Now:

```
== Encoding mostly utf8 (5001001 bytes)
ruby 3.4.0dev (2024-10-18T19:01:45Z master https://github.com/ruby/json/commit/7be9a333ca) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
                json    35.000 i/100ms
                  oj    36.000 i/100ms
           rapidjson    10.000 i/100ms
Calculating -------------------------------------
                json    359.161 (± 1.4%) i/s    (2.78 ms/i) -      1.820k in   5.068542s
                  oj    359.699 (± 0.6%) i/s    (2.78 ms/i) -      1.800k in   5.004291s
           rapidjson     99.687 (± 2.0%) i/s   (10.03 ms/i) -    500.000 in   5.017321s

Comparison:
                json:      359.2 i/s
                  oj:      359.7 i/s - same-ish: difference falls within error
           rapidjson:       99.7 i/s - 3.60x  slower
```

https://github.com/ruby/json/commit/1a338532d2

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