rust: Replace migration README with GitHub issue

The readme was nice to discuss in the implementation PR, but now that this is
merged it's better to have an issue that can be linked against in PRs and
doesn't require further merges to update status.

Ported with a status update in #79975

pkgs/build-support/rust/README.md

@@ -1,45 +0,0 @@
-# Updated fetchCargo behavior
-
-Changes to the `fetchcargo.nix` behavior that cause changes to the `cargoSha256`
-are somewhat disruptive, so historically we've added conditionals to provide
-backwards compatibility. We've now accumulated enough of these that it makes
-sense to do a clean sweep updating hashes, and delete the conditionals in the
-fetcher to simplify maintenance and implementation complexity. These
-conditionals are:
-
-1. When cargo vendors dependencies, it generates a config. Previously, we were
-   hard-coding our own config, but this fails if there are git dependencies. We
-   have conditional logic to sometimes copy the vendored cargo config in, and
-   sometimes not.
-
-2. When a user updates the src package, they may forget to update the
-   `cargoSha256`. We have an opt-in conditional flag to add the `Cargo.lock`
-   into the vendor dir for inspection and compare at build-time, but it defaults
-   to false.
-
-3. We were previously vendoring into a directory with a recursive hash, but
-   would like to vendor into a compressed tar.gz file instead, for the reasons
-   specified in the git commit message adding this feature.
-
-
-## Migration plan
-
-1. (DONE in this PR) Implement `fetchCargoTarball` as a separate, clean fetcher
-   implementation along-side `fetchcargo`. Rename `verifyCargoDeps` (default
-   false) to `legacyCargoFetcher` (default true), which switches the fetcher
-   implementation used. Replace `verifyCargoDeps = true;` with
-   `legacyCargoFetcher = false;` in Rust applications.
-
-2. Send a treewide Rust PR that sets `legacyCargoFetcher = true;` in all Rust
-   applications not using this (which is ~200 of them), with a note to
-   maintainers to delete if updating the package. Change the default in
-   `buildRustPackage` to false.
-
-3. Go through all Rust src packages deleting the `legacyCargoFetcher = false;`
-   line and re-computing the `cargoSha256`, merging as we go.
-
-4. Delete the `fetchcargo.nix` implementation entirely and also remove:
-  - All overrides in application-level packages
-  - The `fetchcargo-default-config.toml` and conditionals around using it when
-    no `$CARGO_CONFIG` exists
-  - This README.md file