Tell Dependabot to keep expectrl and getrandom back for now

This allows them to have patch version updates but not major and
minor version updates. Becuase they are not yet at 1.0.0, this has
the effect of allowing SemVer-compatible but not SemVer-breaking
updates to them via Dependabot version update PRs.

This may be temporary and, in the case of `getrandom`, is intended
to be temporary. For details, see comments in `dependabot.yml`, and:

- https://github.com/GitoxideLabs/gitoxide/pull/2200#issuecomment-3361407300
- https://github.com/GitoxideLabs/gitoxide/pull/2093#issuecomment-3361449228

This also expands the comment for holding back `imara-diff` (all
referenced PRs in the file now have full URLs, not just numbers).

The effect of keeping back `expectrl` was tested and verified to be
the only new hold needed to allow version updates to work, in:
https://github.com/EliahKagan/gitoxide/pull/111

.github/dependabot.yml

@@ -10,7 +10,19 @@ updates:
     allow:
       - dependency-type: all
     ignore:
-      # Keep imara-diff at 0.1.* for now (see comments in #2068).
+      # Keep `expectrl` at 0.7.* for now. See https://github.com/GitoxideLabs/gitoxide/pull/2200.
+      - dependency-name: expectrl
+        update-types:
+          - 'version-update:semver-major'
+          - 'version-update:semver-minor'
+      # Some `getrandom` dependencies are at 0.2.*. Keep them there for now. (Some are at 0.3.*.
+      # This allows them to remain there but keeps them from going to a future 0.4.*.) This is
+      # hopefully temporary. See https://github.com/GitoxideLabs/gitoxide/pull/2093.
+      - dependency-name: getrandom
+        update-types:
+          - 'version-update:semver-major'
+          - 'version-update:semver-minor'
+     # Keep `imara-diff` at 0.1.* for now. See https://github.com/GitoxideLabs/gitoxide/pull/2068.
       - dependency-name: imara-diff
         update-types:
           - 'version-update:semver-major'