use `rust-toolchain.toml` to define toolchain version

[eval-exec]

What problem does this PR solve?

use rust-toolchain.toml to define ckb's rust toolchain version, fix https://github.com/nervosnetwork/ckb/security/dependabot/22

What's Changed:

Related changes

• Delete rust-toolchain file
• Add rust-toolchain.toml file to define ckb's rust toolchain version

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code ci-runs-only: [ quick_checks,linters ]

Side effects

• None

Release note

None: Exclude this PR from the release note.