fix: improve meta migration and warning handling

[vagenas]

Suggests deprecating use_legacy_mode param & deriving whether to use "legacy" annotations (i.e. .annotations) as defined in _should_use_legacy_annotations() (which also takes care of preventing unnecessary warnings).

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[codecov]

Codecov Report

❌ Patch coverage is 75.38462% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
docling_core/transforms/serializer/doctags.py	57.89%	8 Missing ⚠️
docling_core/transforms/serializer/html.py	60.00%	4 Missing ⚠️
docling_core/transforms/serializer/common.py	86.66%	2 Missing ⚠️
docling_core/transforms/serializer/markdown.py	88.88%	1 Missing ⚠️
docling_core/types/doc/document.py	91.66%	1 Missing ⚠️

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