wsgi: use 1.9 config variable name

[pjonsson]

The behavior is different when this variable
contains a list of paths, but that
is not the case here, so just replace
the old variable with the new variable.

---

📚 Documentation preview 📚: https://datacube-ows--1110.org.readthedocs.build/en/1110/

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 93.03%. Comparing base (15d4705) to head (e193238).
Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
datacube_ows/wsgi.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1110   +/-   ##
========================================
  Coverage    93.03%   93.03%           
========================================
  Files           53       53           
  Lines         7080     7080           
========================================
  Hits          6587     6587           
  Misses         493      493           

Files with missing lines	Coverage Δ
datacube_ows/wsgi.py	0.00% <0.00%> (ø)

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