Fix some grid_spec warnings

[pjonsson]

Make a private function that
extracts the same product information
that was previously gotten from
the now deprecated grid_spec,
and then use that helper instead
of grid_spec.

The helper should be removed once
there are suitable replacements
in some part of the opendatacube-API.

---

📚 Documentation preview 📚: https://datacube-explorer--658.org.readthedocs.build/en/658/

[codecov]

Codecov Report

Attention: Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.57%. Comparing base (7d933f4) to head (e4db2ac).
Report is 17 commits behind head on develop.

Files with missing lines	Patch %	Lines
cubedash/summary/_extents.py	96.29%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #658      +/-   ##
===========================================
+ Coverage    89.26%   89.57%   +0.30%     
===========================================
  Files           34       34              
  Lines         4119     4229     +110     
===========================================
+ Hits          3677     3788     +111     
+ Misses         442      441       -1     

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[pjonsson]

This fixes 550 warnings that test_page_load triggers, 220 warnings triggered by test_stac, and a bunch of other warnings.

After this PR, it's clear that the remaining GridSpec warnings are caused by test_summarise_data.py::test_computed_regions_match_those_summarised[default].

[pjonsson]

Making this a draft since opendatacube/datacube-core#1783 revealed the resolution can be a scalar.

If the grid_spec property remains and is no longer deprecated, I think it would be better to use that than replicate the same functionality in explorer.