Replace deprecated datetime.utcnow() in Elexon plotting and tests

[SaitejaKommi]

Description

The Elexon plotting flow and its tests currently use datetime.utcnow(), which is deprecated in Python 3.12+ and emits deprecation warnings.

We should migrate to timezone-aware UTC datetimes (e.g., datetime.now(datetime.UTC)) while preserving the existing behavior for start/end datetime calculations.

This will reduce warning noise and ensure the code remains future-compatible with newer Python versions.

Scope

• elexon_plots.py
• test_elexon_plot.py

Expected Behavior

• No datetime.utcnow() deprecation warnings from these code paths.
• Datetime handling remains functionally equivalent.

Acceptance Criteria

• All datetime.utcnow() usage replaced with timezone-aware alternatives.
• No deprecation warnings from these files.
• Existing Elexon tests continue to pass.
• Changes limited to Elexon code and tests only.

[nightcityblade]

Hi, I'd like to work on this. I'll submit a PR shortly.

[ocf-bot]

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