Avoid using XmlStringBuilder.toString()

[bgrozev]

• chore: Update jitsi-xmpp-extensions.
• fix: Avoid using XmlStringBuilder.toString().

[codecov]

Codecov Report

Merging #529 (b003f56) into master (c386add) will not change coverage.
The diff coverage is 50.00%.

@@            Coverage Diff            @@
##             master     #529   +/-   ##
=========================================
  Coverage     48.13%   48.13%           
  Complexity      174      174           
=========================================
  Files            72       72           
  Lines          2115     2115           
  Branches        201      201           
=========================================
  Hits           1018     1018           
  Misses         1029     1029           
  Partials         68       68           

Files	Coverage Δ
...rc/main/kotlin/org/jitsi/jibri/api/xmpp/XmppApi.kt	49.19% <50.00%> (ø)

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