[Backport 2.1] Bumps version to 2.1.0 on main

[opensearch-trigger-bot[bot]]

Backport 849734da50032f9cf3218017af979442faf6f429 from #129.

[codecov-commenter]

Codecov Report

Merging #134 (32b818c) into 2.1 (9b7fd92) will not change coverage. The diff coverage is n/a.

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@@            Coverage Diff            @@
##                2.1     #134   +/-   ##
=========================================
  Coverage     23.90%   23.90%           
  Complexity     1937     1937           
=========================================
  Files           245      245           
  Lines          6857     6857           
=========================================
  Hits           1639     1639           
  Misses         5218     5218           

Impacted Files	Coverage Δ
src/OpenSearch/Client.php	21.21% <ø> (ø)

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