RUMM-2191: Use ObjectFactory for NamedDomainObjectContainer creation

[0xnm]

What does this PR do?

ObjectFactory methods are not incubating since Gradle 7.0 and they existed since Gradle 4 anyway, so this change implements a switch to this factory for NamedDomainObjectContainer instantiation to get rid of the workaround we had before.

Review checklist (to be filled by reviewers)

• [x] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [x] Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• [x] Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

Merging #87 (8e385d8) into develop (fb392e1) will increase coverage by 0.73%. The diff coverage is 83.33%.

@@              Coverage Diff              @@
##             develop      #87      +/-   ##
=============================================
+ Coverage      90.18%   90.91%   +0.73%     
  Complexity       134      134              
=============================================
  Files             20       20              
  Lines            397      396       -1     
  Branches          45       45              
=============================================
+ Hits             358      360       +2     
+ Misses            24       21       -3     
  Partials          15       15              

Impacted Files	Coverage Δ
...com/datadog/gradle/plugin/DdAndroidGradlePlugin.kt	84.27% <ø> (+0.75%)	:arrow_up:
...in/kotlin/com/datadog/gradle/plugin/DdExtension.kt	85.71% <83.33%> (+19.05%)	:arrow_up:
...in/kotlin/com/datadog/gradle/plugin/DatadogSite.kt	86.67% <0.00%> (+6.67%)	:arrow_up: