RUMM-2209: Prepare version 1.4.0 release

[0xnm]

What does this PR do?

Update changelog and version for the version 1.4.0 release.

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 #89 (c5f37ce) into release/1.4.0 (f1288ce) will increase coverage by 0.45%. The diff coverage is n/a.

@@                 Coverage Diff                 @@
##             release/1.4.0      #89      +/-   ##
===================================================
+ Coverage            89.96%   90.40%   +0.45%     
- Complexity             147      148       +1     
===================================================
  Files                   20       20              
  Lines                  448      448              
  Branches                50       50              
===================================================
+ Hits                   403      405       +2     
+ Misses                  28       27       -1     
+ Partials                17       16       -1     

Impacted Files	Coverage Δ
...in/kotlin/com/datadog/gradle/plugin/DatadogSite.kt	100.00% <0.00%> (+13.33%)	:arrow_up: