doc: doap: Update XEP-0308 version

[melvo]

PR check list:

• [x] Document your code
• [x] Add \since QXmpp 1.X, QXMPP_EXPORT
• [x] Fix doxygen warnings (see log when building with -DBUILD_DOCUMENTATION=ON)
• [x] Update doc/doap.xml
• [x] Add unit tests
• [x] Format the code: Run clang-format -i src/<edited-file(s)> tests/<edited-file(s)>

[codecov[bot]]

Codecov Report

Merging #454 (1905265) into master (74cc776) will increase coverage by 0.00%. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #454   +/-   ##
=======================================
  Coverage   68.56%   68.56%           
=======================================
  Files         267      267           
  Lines       23607    23608    +1     
=======================================
+ Hits        16187    16188    +1     
  Misses       7420     7420           

Impacted Files	Coverage Δ
tests/qxmppomemodata/tst_qxmppomemodata.cpp	98.25% <0.00%> (+<0.01%)	:arrow_up:

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