Fix for datetime deprecation and standardizing timezone and isoformat (Issue #2327)

[imperorrp]

Fix for datetime deprecation issue and standardizing timezone(UTC) and format(isoformat) for datetime updates on concatenating InferenceData (issue #2327 )

Description

The "created_at" default attribute in the make_attrs function in arviz/data/base.py is updated to avoid deprecation as well as work for Python 3.10, which is still supported by ArviZ.

The current_time variable in the concat function in arviz/data/inference-data.py is also updated to use the UTC timezone and is isoformatted.

Checklist

• Follows official PR format
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• New features are properly documented (with an example if appropriate)?
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

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📚 Documentation preview 📚: https://arviz--2329.org.readthedocs.build/en/2329/

[OriolAbril]

The failing test is unrelated, due to a deprecation in scipy.signal. Next release will remove gaussian function from there so we should fix that soonish too. We can merge this already though