fix(useMounted): compatible with vue2.7

[yanbowe]

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⚠️ Slowing down new functions

Warning: Slowing down new functions

As the VueUse audience continues to grow, we have been inundated with an overwhelming number of feature requests and pull requests. As a result, maintaining the project has become increasingly challenging and has stretched our capacity to its limits. As such, in the near future, we may need to slow down our acceptance of new features and prioritize the stability and quality of existing functions. Please note that new features for VueUse may not be accepted at this time. If you have any new ideas, we suggest that you first incorporate them into your own codebase, iterate on them to suit your needs, and assess their generalizability. If you strongly believe that your ideas are beneficial to the community, you may submit a pull request along with your use cases, and we would be happy to review and discuss them. Thank you for your understanding.

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Description

fix #3803

Additional context

[Glandos]

Indeed, the Typescript written in https://github.com/vuejs/vue/blob/bed04a77e575d6c4c1d903f60087dca874f7213e/src/v3/apiLifecycle.ts#L6 results in the following generated code:

function createLifeCycle(hookName) {
    return function (fn, target) {
        if (target === void 0) { target = currentInstance; }
        if (!target) {
            process.env.NODE_ENV !== 'production' &&
                warn("".concat(formatName(hookName), " is called when there is no active component instance to be ") +
                    "associated with. " +
                    "Lifecycle injection APIs can only be used during execution of setup().");
            return;
        }
        return injectHook(target, hookName, fn);
    };
}

Since target is strictly checked against undefined (aka void 0), the modification here doesn't work. #3830 is the way to go, albeit using undefined for better readability.