Improve asciidoctor configuration in build

[lhotari]

• use recent versions that work
• move asciidoctor related config to separate file from build.gradle

[codecov-io]

Codecov Report

Merging #348 into master will decrease coverage by 0.34%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #348      +/-   ##
============================================
- Coverage     69.35%   69.01%   -0.35%     
+ Complexity     1051     1046       -5     
============================================
  Files            72       72              
  Lines          4386     4386              
  Branches        628      628              
============================================
- Hits           3042     3027      -15     
- Misses          964      977      +13     
- Partials        380      382       +2

Impacted Files	Coverage Δ	Complexity Δ
...tor/ipc/netty/channel/CloseableContextHandler.java	52.63% <0%> (-5.27%)	8% <0%> (-1%)
.../ipc/netty/channel/PooledClientContextHandler.java	63.02% <0%> (-5.05%)	25% <0%> (-2%)
src/main/java/reactor/ipc/netty/FutureMono.java	67.5% <0%> (-1.67%)	8% <0%> (ø)
...a/reactor/ipc/netty/channel/ChannelOperations.java	80.46% <0%> (-1.57%)	46% <0%> (-1%)
...or/ipc/netty/http/server/HttpServerOperations.java	67.19% <0%> (-0.53%)	53% <0%> (-1%)
...or/ipc/netty/channel/ChannelOperationsHandler.java	66.81% <0%> (-0.45%)	60% <0%> (ø)

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