Change Bulgaria currency unit

[gewarren]

Fixes #12197.

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Tagging subscribers to this area: @dotnet/area-system-globalization

[Copilot]

Pull request overview

This PR updates the RegionInfo API documentation, including currency information and general formatting, to reflect current data and improve consistency, notably updating Bulgaria’s currency to EUR per issue #12197.

Changes:

• Updated the RegionInfo.ISOCurrencySymbol table entry for Bulgaria from “Bulgarian Lev (BGN)” to “Euro (EUR)” and refreshed the surrounding ISO 4217 description/link.
• Normalized many remarks sections by removing redundant “## Remarks” headings, adjusting paragraph formatting, and standardizing example/code block introductions.
• Refined various documentation details (for example, wording around NativeName, DisplayName, and CurrentRegion) for consistency across the RegionInfo members.