Fix compatibility with DTLS 1.3.

[JonathanLennox]

Remove lingering pieces of DTLS 1.0 support.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9d121e4) 43.96% compared to head (d3de1d7) 44.26%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2086      +/-   ##
============================================
+ Coverage     43.96%   44.26%   +0.30%     
- Complexity     1920     1928       +8     
============================================
  Files           340      340              
  Lines         18922    18908      -14     
  Branches       2595     2593       -2     
============================================
+ Hits           8319     8370      +51     
+ Misses         9754     9678      -76     
- Partials        849      860      +11     

Files	Coverage Δ
...rc/main/kotlin/org/jitsi/nlj/dtls/TlsClientImpl.kt	65.62% <100.00%> (-1.05%)	⬇️
...rc/main/kotlin/org/jitsi/nlj/dtls/TlsServerImpl.kt	57.14% <100.00%> (-1.40%)	⬇️

... and 3 files with indirect coverage changes

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