Reduce number of Groovy dependencies to just groovy.jar

[szpak]

Spock shouldn't provide the other to do not pollute the classpath.
It's common practice nowadays to exclude transitive dependencies,
which is not good as "spock-groovy2-compat" is needed with Groovy 2.

Proposed realase notes entry:

=== Breaking Changes

==== Reduce number of transitive Groovy dependencies to just groovy.jar

Spock started to provide only one Groovy dependency - `groovy.jar`. The others,
in the majority of cases were not needed in the end projects and were only polluting classpath
(or excluded explicitly). If other Groovy dependencies are needed in the project,
they have to be provided explicitly.

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This change is 

[codecov]

Codecov Report

Merging #1109 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1109   +/-   ##
=========================================
  Coverage     74.24%   74.24%           
  Complexity     3472     3472           
=========================================
  Files           385      385           
  Lines         10661    10661           
  Branches       1309     1309           
=========================================
  Hits           7915     7915           
  Misses         2285     2285           
  Partials        461      461

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[Vampire]

-has +have in the release notes

[Vampire]

lgtm