fix: make no-concurrency mode default

[supercaracal]

ruby 3.3.6 (2024-11-05 revision 75015d4c1f) [x86_64-linux]
Warming up --------------------------------------
 pipelined: ondemand    72.000 i/100ms
   pipelined: pooled    82.000 i/100ms
     pipelined: none    84.000 i/100ms
    pipelined: envoy    80.000 i/100ms
   pipelined: cproxy    39.000 i/100ms
Calculating -------------------------------------
 pipelined: ondemand    715.452 (± 6.6%) i/s    (1.40 ms/i) -      3.600k in   5.064794s
   pipelined: pooled    830.765 (± 0.7%) i/s    (1.20 ms/i) -      4.182k in   5.034200s
     pipelined: none    846.257 (± 5.8%) i/s    (1.18 ms/i) -      4.284k in   5.084215s
    pipelined: envoy    805.275 (± 7.0%) i/s    (1.24 ms/i) -      4.000k in   5.007049s
   pipelined: cproxy    400.326 (± 5.2%) i/s    (2.50 ms/i) -      2.028k in   5.085483s

Comparison:
     pipelined: none:      846.3 i/s
   pipelined: pooled:      830.8 i/s - same-ish: difference falls within error
    pipelined: envoy:      805.3 i/s - same-ish: difference falls within error
 pipelined: ondemand:      715.5 i/s - 1.18x  slower
   pipelined: cproxy:      400.3 i/s - 2.11x  slower

The thread-initialization cost is too expensive against a standart-scale service.