Use wp_is_rest_endpoint() to detect if we are handling a REST API request

[mukeshpanchal27]

Summary

Follow-up to #1206 implementation:

We bumped the WordPress minimum version to 6.6 in #1683, so in perflab_rest_post_dispatch_add_server_timing(), we can now safely use the core WordPress function wp_is_rest_endpoint () mentioned in the TODO comment.

In the PR i update the code to use that function and remove the TODO.

Relevant technical choices

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 67.18%. Comparing base (e6f4f18) to head (e2c565c).
Report is 3 commits behind head on trunk.

Files with missing lines	Patch %	Lines
...s/performance-lab/includes/server-timing/hooks.php	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk    #2094   +/-   ##
=======================================
  Coverage   67.18%   67.18%           
=======================================
  Files          93       93           
  Lines        7750     7750           
=======================================
  Hits         5207     5207           
  Misses       2543     2543           

Flag	Coverage Δ
multisite	67.18% <0.00%> (ø)
single	36.61% <0.00%> (ø)

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Co-authored-by: mukeshpanchal27 <[email protected]>
Co-authored-by: westonruter <[email protected]>
Co-authored-by: b1ink0 <[email protected]>

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