Use HTTPS for the clearadi submodule URL

[gregakespret]

The clearadi submodule is declared with an SSH URL
(git@github.com:OpenBubbles/clearadi-stub.git). That breaks
cargo build for any downstream crate that depends on this repo
transitively, because Cargo's URL parser rejects the SSH form before
any fetch is attempted — and importantly, even setting
net.git-fetch-with-cli = true doesn't help, because URL parsing
happens before the fetch is delegated to system git.

Concrete failure mode (reproduced against current master via
thisiscam/rustpush → OpenBubbles/apple-private-apis):

$ cargo build --release
error: failed to update submodule `apple-private-apis`

Caused by:
  failed to update submodule `clearadi`

Caused by:
  invalid url `git@github.com:OpenBubbles/clearadi-stub.git`:
  relative URL without a base; try using
  `ssh://git@github.com/OpenBubbles/clearadi-stub.git` instead

Since clearadi-stub is a public read-only repository, an HTTPS URL
is functionally equivalent for everyone and removes this friction
entirely. Anyone who prefers SSH for git operations can override
locally without affecting downstream crates:

git config url."git@github.com:".insteadOf "https://github.com/"

How was this tested

Cloned this repo recursively via system git after applying the change
— clearadi-stub checks out cleanly without any authentication step.
Then built a downstream crate (thisiscam/export-findmy →
thisiscam/rustpush) against this commit and confirmed cargo build
no longer errors on the submodule URL.