`use_repo` should be called `use_repos`

### Description of the bug:

The name `use_repo` doesn't make it clear that the function accepts an arbitrary number of arguments. There are examples of BCR modules with successive, repeated `use_repo` calls with a single repo name for the same extension proxy, so this is already providing incorrect intuition to real users.

The singular form is also inconsistent with other API such as `register_toolchains` and `register_execution_platforms`.

Since there are only ~40 modules today, many of which don't even use `use_repo`, adding `use_repos` and keeping `use_repo` as a deprecated alias for the time being may be doable. Given that this new function will (hopefully) stay with us for a long time, it may very well be worth the effort.

### What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

_No response_

### Which operating system are you running Bazel on?

Any

### What is the output of `bazel info release`?

6.0.0

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse master; git rev-parse HEAD` ?

_No response_

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

_No response_

Start by locating the definition and call sites of use_repo in Bazel's module-extension API, then compare its behavior and naming with register_toolchains and register_execution_platforms. Audit the roughly 40 BCR modules to identify migration needs; done means use_repos accepts the intended arguments while use_repo remains a deprecated alias and affected examples continue to work.

Written by the indexing model from the issue text.