Fix deprecation warnings

[SpacemanPaul]

Reason for this pull request

Some upstream libraries were raising deprecation warnings from core, as reported in #1474

Proposed changes

• Update SQLAlchemy calls to suppress remaining deprecation warnings.

• Pin jsonschema<4.18 to suppress deprecation warnings from old (RefResolver) API. (Move to the new API will be done in a separate PR, to be merged once jsonschema 4.18+ has stabilised.)

• Closes Deprecation Warnings from jsonschema and SQLAlchemy  #1474

• Tests added / passed

• Fully documented, including docs/about/whats_new.rst for all changes

---

📚 Documentation preview 📚: https://datacube-core--1476.org.readthedocs.build/en/1476/

[omad]

lgtm. Tested to confirm it fixes the warnings I had been getting.

[codecov]

Codecov Report

Patch coverage: 90.62% and no project coverage change.

Comparison is base (95b6f43) 91.78% compared to head (ac52a4a) 91.78%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1476   +/-   ##
========================================
  Coverage    91.78%   91.78%           
========================================
  Files          132      132           
  Lines        14526    14530    +4     
========================================
+ Hits         13333    13337    +4     
  Misses        1193     1193           

Impacted Files	Coverage Δ
datacube/drivers/postgres/_core.py	96.29% <87.50%> (+0.03%)	⬆️
datacube/drivers/postgis/_api.py	87.31% <100.00%> (+0.03%)	⬆️
datacube/drivers/postgis/_core.py	95.04% <100.00%> (+0.10%)	⬆️
datacube/index/postgis/_datasets.py	74.81% <100.00%> (ø)

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