Fix premature connection closure

[Ariana-B]

Reason for this pull request

On servers that aggressively close idle connections (e.g. NCI), the connection is sometimes closed prematurely when combining db calls with file io or writing to logs.

Proposed changes

• Connection.execution_options doesn't modify the connection in-place until sqlalchemy 2.0; make current implementation compatible with both 1.4 and 2.0

• Don't set connection isolation level to autocommit when calling _connect, as it is already set on the engine level

• [ ] 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--1525.org.readthedocs.build/en/1525/