chore: pin toolchain to MSRV(1.76)

[SteveLauC]

What does this PR do

1. Pin the toolchain to 1.76.0
2. For our CI, remove the dtolnay/rust-toolchain action as it is no longer needed:
   1. GH CI provides rustup by default
   2. We have the toolchain specified in rust-toolchain.toml
   3. Thus, cargo xxx will download the toolchain automatically

Standards checklist

• The PR title is descriptive.
• I have read CONTRIBUTING.md
• Optional: I have tested the code myself

For new steps

• Optional: Topgrade skips this step where needed
• Optional: The --dry-run option works with this step
• Optional: The --yes option works with this step if it is supported by
  the underlying command

If you developed a feature or a bug fix for someone else and you do not have the
means to test it, please tag this person here.