docs(ios): refresh README for Xcode 15 and modern targets

[hechibing]

Summary

• update iOS bindings README from Xcode 13-era instructions to Xcode 15+
• document required Rust iOS targets and target-specific artifact paths
• refresh xcodebuild commands and linker-path guidance for simulator/device builds

Validation

• verified README steps align with current Config/*.xcconfig target paths

Closes #759

[coveralls]

Pull Request Test Coverage Report for Build 22015522344

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 5 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.01%) to 86.907%

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Files with Coverage Reduction	New Missed Lines	%
ergo-chain-types/src/header.rs	1	99.26%
ergo-lib/src/wallet/tx_builder.rs	4	93.66%

Totals
Change from base Build 21705206395:	0.01%
Covered Lines:	27473
Relevant Lines:	31612

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💛 - Coveralls