[border-agent] use new otBorderAgent ephemeral key APIs

[abtink]

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Depends-On: openthread/openthread#11166

[codecov]

Codecov Report

Attention: Patch coverage is 4.34783% with 22 lines in your changes missing coverage. Please review.

Project coverage is 43.36%. Comparing base (2b41187) to head (15b547a).
Report is 934 commits behind head on main.

Files with missing lines	Patch %	Lines
src/border_agent/border_agent.cpp	7.69%	11 Missing and 1 partial ⚠️
src/dbus/server/dbus_thread_object_rcp.cpp	0.00%	10 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2674       +/-   ##
===========================================
- Coverage   55.77%   43.36%   -12.42%     
===========================================
  Files          87      108       +21     
  Lines        6890    13369     +6479     
  Branches        0      961      +961     
===========================================
+ Hits         3843     5797     +1954     
- Misses       3047     7265     +4218     
- Partials        0      307      +307     

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