Merge bitcoindevkit/bdk#2330: docs(electrum): drop stale MSRV section from README

Merge bitcoindevkit/bdk#2330: docs(electrum): drop stale MSRV section from README

04853e31e63ab75c11395ecc4d9d248d265a797b docs(electrum): drop stale MSRV section from README (artofbitcoin)

Pull request description:

  ### Description

  Remove the stale MSRV section from `crates/electrum/README.md`, which still claims Rust 1.75.0 and includes an obsolete dependency pinning command.

  The root README already documents the current MSRV and pinning instructions. Removing the duplicate section reduces maintenance and makes the Electrum README consistent with the other crate READMEs.

  ### Checklists

  #### All Submissions:

  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

ACKs for top commit:
  noahjoeris:
    ACK 04853e31e63ab75c11395ecc4d9d248d265a797b

Tree-SHA512: 45c82a13dd0a03540e6e719a1a9caff18cf64ff22ab7bb3601a5d8eb64ca489dd64f2cad0a1dec90aaad3304f3620b6599e24d48a462f677924551429baa3963