Automatically migrate `.replace` to `.replaceChildren` for assertionTemplates

The `.replace` API has been renamed in version 6 to `replaceChildren`. We should look at whether this is something that we can automate using the upgrade tool.

```ts
const baseAssertion = assertionTemplate(() => v('div', { '~key': 'root' }, [
v('span')
]));

const buttonAssertion = baseAssertion.replace('~root', [ v('button') ]);

const otherAssertion = buttonAssertion.replace('~root', [ v('div') ]);
```

Would become

```ts
const baseAssertion = assertionTemplate(() => v('div', { '~key': 'root' }, [
v('span')
]));

const buttonAssertion = baseAssertion.replaceChildren('~root', [ v('button') ]);

const otherAssertion = buttonAssertion.replaceChildren('~root', [ v('div') ]);
```

No file or test is named. Start by tracing the upgrade tool's migration entry points and existing transforms for assertionTemplates, then determine whether the example's `.replace` calls can be safely rewritten to `.replaceChildren`. Done means the upgrade tool performs this migration correctly and has coverage for the shown transformation.

Written by the indexing model from the issue text.