[Backport 2.x] dependabot: bump com.diffplug.spotless from 6.20.0 to 6.21.0

[opensearch-trigger-bot]

Backport 7c61fd9 from #3295.

[codecov]

Codecov Report

Merging #3298 (fcf8370) into 2.x (d8a30d4) will increase coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##                2.x    #3298   +/-   ##
=========================================
  Coverage     63.13%   63.13%           
- Complexity     3343     3345    +2     
=========================================
  Files           256      256           
  Lines         19441    19441           
  Branches       3282     3282           
=========================================
+ Hits          12274    12275    +1     
+ Misses         5573     5571    -2     
- Partials       1594     1595    +1     

see 1 file with indirect coverage changes

[cwperks]

• What went wrong:
  Execution failed for task ':bundlePlugin'.
  Could not resolve all dependencies for configuration ':runtimeClasspath'.
  Conflict found for the following module:
  - org.apache.commons:commons-lang3 between versions 3.13.0 and 3.12.0

[cwperks]

org.apache.commons:commons-lang3:3.13.0
\--- org.opensearch:opensearch-encryption-sdk:2.10.0-SNAPSHOT
     \--- org.opensearch:opensearch:2.10.0-SNAPSHOT
          \--- org.opensearch.client:opensearch-rest-high-level-client:2.10.0-SNAPSHOT
               \--- runtimeClasspath

org.apache.commons:commons-lang3:3.12.0 -> 3.13.0
\--- runtimeClasspath

[cwperks]

I opened up a PR to resolve the failing check on this PR: #3306