Use shields.io's MSRV badges

[Sky9x]

Added in https://github.com/badges/shields/pull/9871

Updated:

• embedded-hal
• embedded-hal-bus
• embedded-hal-nb: old badge said 1.60 but rust-version in Cargo.toml is set to 1.56

Skipped:

• embedded-can: rust-version field is blank on crates.io

No Badge (MSRV is in readme):

• embedded-hal-async
• embedded-io-adapters
• embedded-io-async
• embedded-io

(I can add a badge for these if desired)