style/standardize solidity capitalization

[Earnwithalee7890]

What changed? Why?
Capitalized "solidity" to "Solidity" when referring to the programming language in documentation text.
Proper capitalization of language names (like TypeScript, JavaScript, Solidity) is a standard technical writing convention and improves professional consistency.

Notes to reviewers

• Updated instances in learning-objectives.mdx ("common Solidity errors") and structs-sbs.mdx (error message headers like "from Solidity:").
• Only modified text references to the language name, not code blocks, file paths, or URLs.
• Lowercase solidity in code fences (e.g., ```solidity) remains unchanged as it's a syntax identifier.

How has it been tested?

• grep search to identify all lowercase "solidity" instances in text.
• Manual verification that changes were limited to natural language references.
• Confirmed that code block language identifiers and URLs were not affected.

[cb-heimdall]

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