Rename `SEMANTIC_ATTRIBUTE_SENTRY_SOURCE` to `sentry.span.source` & drop `semanticAttributes` re-export

[linear-code]

Rename the internal SEMANTIC_ATTRIBUTE_SENTRY_SOURCE attribute to sentry.span.source, remove the backfill kept for back-compat, and drop the deprecated semanticAttributes re-export.

TODOs:

• packages/core/src/tracing/spans/captureSpan.ts:88 — remove backfill once renamed to sentry.span.source
• packages/core/src/tracing/dynamicSamplingContext.ts:141 — only read SENTRY_SPAN_SOURCE once SEMANTIC_ATTRIBUTE_SENTRY_SOURCE is removed
• packages/core/src/semanticAttributes.ts:4 — remove this export

[linear-code]

JS-3039