Remove bors-ng and switch to GitHub actions merge-queue

[lschuermann]

This removes the bors configuration and switches to the GitHub actions merge queue merge bot. It replicates the required status checks for bors to pass, and adds the appropriate GitHub merge group status checks. This PR needs to be followed up with the appropriate configuration applied in the repository settings.

[lschuermann]

In the interest of transparency, I made the following changes to this repository's configuration:

• Created a branch protection rule for master, with the following settings:
  • Require a pull request before merging
    • Require approvals: at least 1 approval
    • Dismiss stale pull request approvals when new commits are pushed
  • Require status checks to pass before merging
    • Can't select any status checks as there have been no statuses pushed in the last week, will update, should be:
    • ci
    • size-diff
    • This replicates the bors.toml-required statuses

After this is merged, I will make the following further adjustments:

• Select the required status as described above
• Require the merge queue
  • Using merge commits
  • Only merging non-failing pull requests

[lschuermann]

Merging this with permission by @jrvanwhy in #476 (comment). However, CI is failing because of an update in elf2tab, fixing that by bumping the Rust toolchain version in #481 switching to the stable toolchain for elf2tab in #486.