Improve archive_less_mature compatibility with odc-tools

[Ariana-B]

Reason for this pull request

odc-tools also uses the archive_less_mature functionality, but additionally needs to be able to access the less mature datasets to public the STAC document to an SNS topic. With the current implementation of archive_less_mature, this requires a lot of duplicate code.

Proposed changes

• Split the search logic in archive_less_mature into a new function, find_less_mature, that simply returns a list of the less mature Datasets

• Allow a leniency of 1ms in searching for other maturity versions of a dataset

• Closes #xxxx

• Tests added / passed

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

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📚 Documentation preview 📚: https://datacube-core--1452.org.readthedocs.build/en/1452/

[codecov]

Codecov Report

Patch coverage: 90.00% and no project coverage change.

Comparison is base (b34d375) 91.82% compared to head (c6f73fe) 91.82%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1452   +/-   ##
========================================
  Coverage    91.82%   91.82%           
========================================
  Files          132      132           
  Lines        14510    14516    +6     
========================================
+ Hits         13324    13330    +6     
  Misses        1186     1186           

Impacted Files	Coverage Δ
datacube/index/fields.py	96.77% <ø> (ø)
datacube/index/abstract.py	88.28% <90.00%> (-0.05%)	⬇️

... and 1 file with indirect coverage changes

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[SpacemanPaul]

Actually - can we round the time off to the nearest millisecond, as requested by James Miller in SnS #1188