docs(balances): fix broken links and correct trait name in README

[CECILIA-MULANDI]

Description

Fixes broken documentation links in the Balances module README and corrects the trait name from IsDeadAccount (which doesn't exist) to OnKilledAccount (the correct trait). Also adds deprecation warnings for Currency-related traits to guide developers to use the newer fungible traits instead.

Integration

No integration needed - this is a documentation-only change. The R0-no-crate-publish-required label applies to this PR.

Review Notes

This PR fixes documentation issues in substrate/frame/balances/README.md:

• Fixed broken links: Added proper markdown links with URLs to trait documentation (e.g., [Currency](https://docs.rs/frame-support/...))
• Corrected trait name: Changed IsDeadAccount → OnKilledAccount. Verified that OnKilledAccount is the correct trait name and IsDeadAccount does not exist in the codebase
• Added deprecation warnings: Added deprecation notices (⚠️ Deprecated) for Currency, ReservableCurrency, and LockableCurrency traits with a note directing developers to use the fungible traits instead

No code changes, only documentation improvements.

Checklist

• My PR includes a detailed description as outlined in the "Description" and its two subsections above.
• My PR follows the labeling requirements of this project (at minimum one label for T required)
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• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[cla-bot-2021]

User @CECILIA-MULANDI, please sign the CLA here.