fix flag for ignoring missing geothermal data in non-EU countries

[amos-schledorn]

Fixes the config flag for ignoring missing (non-EU) regions in ISI geothermal heat source data.

Changes proposed in this Pull Request

• introduce config setting sector:district_heating:ignore_missing_geothermal_data
• updated naming of flag in build_sector.smk and build_geothermal_heat_potentials.py
• removed default value false in build_geothermal_heat_potentials.py

Checklist

Required:

• Changes are tested locally and behave as expected.
• Code and workflow changes are documented.
• A release note entry is added to doc/release_notes.rst.

If applicable:

• Changes in configuration options are reflected in scripts/lib/validation.
• For new data sources or versions, these instructions have been followed.
• New rules are documented in the appropriate doc/*.rst files.