Increment version to 2.7

[lezzago]

Issue #, if available:
N/A

Description of changes:
Increment version to 2.7

CheckList:
[X] Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
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[codecov-commenter]

Codecov Report

Merging #823 (fb80a8b) into 2.x (fc1ecb0) will increase coverage by 0.10%.
The diff coverage is n/a.

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@@             Coverage Diff              @@
##                2.x     #823      +/-   ##
============================================
+ Coverage     75.59%   75.69%   +0.10%     
  Complexity      116      116              
============================================
  Files           125      125              
  Lines          6868     6868              
  Branches       1031     1031              
============================================
+ Hits           5192     5199       +7     
+ Misses         1152     1143       -9     
- Partials        524      526       +2     

Impacted Files	Coverage Δ
...destinationmigration/DestinationConversionUtils.kt	71.11% <0.00%> (+1.11%)	⬆️
...in/org/opensearch/alerting/MonitorRunnerService.kt	82.00% <0.00%> (+2.00%)	⬆️
...otlin/org/opensearch/alerting/alerts/AlertMover.kt	70.37% <0.00%> (+7.40%)	⬆️

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