Fix read/write of common.Contact

[khaeru]

• Attach com:Name to the Agency, not the Contact.
• Write URI and e-mail as element text, not an attribute named "text".

PR checklist

• Checks all ✅
• Update documentation Bug fix
• Update doc/whatsnew.rst

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f0e00bc) 98.72% compared to head (c425a6e) 96.51%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #159      +/-   ##
==========================================
- Coverage   98.72%   96.51%   -2.21%     
==========================================
  Files          87       87              
  Lines        7043     7055      +12     
==========================================
- Hits         6953     6809     -144     
- Misses         90      246     +156     

Files	Coverage Δ
sdmx/reader/xml/v21.py	98.83% <100.00%> (-0.21%)	⬇️
sdmx/tests/reader/test_reader_xml_v21.py	100.00% <100.00%> (ø)
sdmx/tests/writer/test_writer_xml.py	100.00% <ø> (ø)
sdmx/writer/xml.py	97.68% <ø> (ø)

... and 17 files with indirect coverage changes