Change Default DataGateway API Backend to Python ICAT

[VKTB]

This PR will close #318

Description

Setting Python ICAT as the default backend since that is the backend that is used in production.

Testing Instructions

• Review code
• Check GitHub Actions build
• If icatdb Generator Script Consistency Test CI job fails, is this because of a deliberate change made to the script to change generated data (which isn't actually a problem) or is here an underlying issue with the changes made?
• Review changes to test coverage
• Does this change mean a new patch, minor or major version should be made? If so, does one of the commit messages feature fix:, feat: or BREAKING CHANGE: so a release is automatically made via GitHub Actions upon merge?

[codecov]

Codecov Report

Merging #334 (f15522f) into master (11d59e8) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #334      +/-   ##
==========================================
+ Coverage   92.62%   92.69%   +0.06%     
==========================================
  Files          43       43              
  Lines        3215     3216       +1     
  Branches      321      321              
==========================================
+ Hits         2978     2981       +3     
  Misses        198      198              
+ Partials       39       37       -2     

Impacted Files	Coverage Δ
datagateway_api/src/search_api/helpers.py	97.46% <0.00%> (-2.54%)	⬇️
...gateway_api/src/search_api/query_filter_factory.py	95.23% <0.00%> (-0.32%)	⬇️
datagateway_api/src/common/config.py	100.00% <0.00%> (ø)
...agateway_api/src/resources/search_api_endpoints.py	100.00% <0.00%> (ø)
datagateway_api/src/api_start_utils.py	86.29% <0.00%> (+0.45%)	⬆️
datagateway_api/src/search_api/models.py	98.50% <0.00%> (+1.80%)	⬆️

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