Rename property in gradle plugin extension from input to source

[marschwar]

This closes #3918

Since switching to org.gradle.api.tasks.SourceTask, there is a difference how users would specify the input to be checked using the detekt extension vs. creating a custom task which seemed to have confused users.

detekt {
    input = files(
        "src/main/kotlin",
        "gensrc/main/kotlin"
    )
}

vs.

tasks.register(name: myDetekt, type: io.gitlab.arturbosch.detekt.Detekt) {
   source = files("src/main/kotlin", "src/test/kotlin")
}

[codecov]

Codecov Report

Merging #3951 (be4d00e) into main (d81d6a2) will decrease coverage by 0.13%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main    #3951      +/-   ##
============================================
- Coverage     83.58%   83.44%   -0.14%     
- Complexity     3123     3149      +26     
============================================
  Files           456      456              
  Lines          9014     9014              
  Branches       1752     1754       +2     
============================================
- Hits           7534     7522      -12     
- Misses          564      565       +1     
- Partials        916      927      +11     

Impacted Files	Coverage Δ
...sch/detekt/rules/coroutines/SleepInsteadOfDelay.kt	72.00% <0.00%> (-8.00%)	⬇️
...arturbosch/detekt/generator/collection/KDocTags.kt	62.50% <0.00%> (-5.00%)	⬇️
...h/detekt/rules/style/DataClassContainsFunctions.kt	95.00% <0.00%> (-5.00%)	⬇️
...rturbosch/detekt/rules/bugs/UnnecessarySafeCall.kt	73.91% <0.00%> (-4.35%)	⬇️
...io/github/detekt/report/sarif/SarifOutputReport.kt	90.90% <0.00%> (-3.04%)	⬇️
...rturbosch/detekt/rules/style/UnusedPrivateClass.kt	70.66% <0.00%> (-2.67%)	⬇️
...ekt/generator/collection/DocumentationCollector.kt	85.00% <0.00%> (-2.50%)	⬇️
...t/rules/style/UtilityClassWithPublicConstructor.kt	93.61% <0.00%> (-2.13%)	⬇️
...ain/kotlin/io/gitlab/arturbosch/detekt/cli/Spec.kt	88.23% <0.00%> (-1.97%)	⬇️

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[schalkms]

I favor this change compared to the old behavior.

[cortinico]

Are we ok to merge this?