Fix code scanning alert no. 21: Database query built from user-controlled sources

[harsh6754]

Fixes https://github.com/harsh6754/Solar-Power-Application/security/code-scanning/21

To fix the problem, we need to ensure that the user-provided data in req.body is sanitized before it is used in the MongoDB query. One way to achieve this is by validating the update object to ensure it only contains expected fields and values. We can use a library like Joi for schema validation.

1. Install the Joi library for schema validation.
2. Define a schema for the update object that specifies the allowed fields and their types.
3. Validate the update object against the schema before using it in the query.
4. If the validation fails, return an error response.

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