[3rdparty] Bump phpseclib to 2.0.4

[rullzer]

See: https://github.com/phpseclib/phpseclib/blob/master/CHANGELOG.md

[MorrisJobke]

Updated the submodule, because it was merged.

[codecov-io]

Codecov Report

Merging #4139 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master   #4139      +/-   ##
===========================================
- Coverage      54.2%   54.2%   -0.01%     
  Complexity    21350   21350              
===========================================
  Files          1312    1312              
  Lines         81424   81424              
  Branches       1285    1285              
===========================================
- Hits          44135   44133       -2     
- Misses        37289   37291       +2

Impacted Files	Coverage Δ	Complexity Δ
lib/private/Files/Cache/Propagator.php	94.93% <0%> (-1.27%)	16% <0%> (ø)
apps/files_trashbin/lib/Trashbin.php	73.39% <0%> (-0.25%)	133% <0%> (ø)

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[rullzer]

CI is happy! Review time.