Remove usage of deprecated serde_if_integer128 and bump serde dependecy

[Mingun]

Serde 1.0.221 deprecated serde_if_integer128 macro, which leads to warnings when compile quick-xml. That macro does nothing since serde 1.0.187, and since 1.0.180 serde requires compiler >=1.26 which stabilizes 128-bit integers.

So it seems that it is enough to require serde >=1.0.180

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 58.08%. Comparing base (38b44d4) to head (d1acdb5).
⚠️ Report is 78 commits behind head on master.

Files with missing lines	Patch %	Lines
src/de/mod.rs	0.00%	1 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #908      +/-   ##
==========================================
+ Coverage   55.52%   58.08%   +2.55%     
==========================================
  Files          42       42              
  Lines       15511    15513       +2     
==========================================
+ Hits         8613     9011     +398     
+ Misses       6898     6502     -396     

Flag	Coverage Δ
unittests	58.08% <0.00%> (+2.55%)	⬆️

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[Mingun]

I do not like to bump dependencies in patch releases so hold this until minor release. I did not look at #904 yet, so if it go into patch, I'll merge it first.