Refine Dependabot config to not update rust-toolchain

When `dtolnay/rust-toolchain` is a tag, it specifies a particular
version of the actual Rust toolchain to use. It is also sometimes
used with a branch such as `master` (to then specify the version as
an option rather than in the action version) or `stable`, but
branches are ineligible for Dependabot version updates and also
would not ordinarily require them since they move much more often
than tags.

Therefore, this tells Dependabot to ignore `dtolnay/rust-toolchain`
for the purpose of version updates. As commented, this is done in a
way that *should* avoid causing it to be ignored for security
updates.

Old Rust toolchains may have known vulnerabilities but still be
safe to use on CI based on a consideration of the risks (for
triggers such as `push` and `pull_request` that run with the same
privileges of the user who can cause the triggering event to
occur), and I believe the action itself does not get advisories for
those versions. But it is possible that at some point in the future
a vulnerability might be discovered in the action itself and an
advisory produced for it, so having Dependabot be able to show any
related alert and, if enabled, produce a security update PR for it
automatically is still valuable.

Ignoring `dtolnay/rust-toolchain` addresses the problem identified
in GitoxideLabs#1362 (review)
without requiring any decreased use of actions to install
dependencies, modification of how the actions' versions or options
are given, or `@dependabot` ignore commands.

.github/dependabot.yml

@@ -4,6 +4,16 @@ updates:
  directory: "/"
  schedule:
  interval: weekly
+ ignore:
+ # Ignore version updates to dtolnay/rust-toolchain, as @X.Y.Z tags are used for exact toolchain
+ # versions (and @master and @stable are branches). To still get Dependabot *security* updates
+ # if the action itself ever has an advisory, we list all version update types explicitly. See:
+ # https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
+ dependency-name: dtolnay/rust-toolchain
+ update-types:
+ - "version-update:semver-major"
+ - "version-update:semver-minor"
+ - "version-update:semver-patch"
  groups:
  github-actions:
  patterns: ["*"]