Replace all occurrences of a substring within a string with another string, and return the number of replacements made.

Examples:

Input: text = "Salesforce is a great platform. Salesforce has many features." oldSubstring = "Salesforce" newSubstring = "Apex" Output: 2 Explanation: "Salesforce" is replaced with "Apex" twice, resulting in "Apex is a great platform. Apex has many features." Input: text = "Hello world" oldSubstring = "xyz" newSubstring = "abc" Output: 0 Explanation: No replacements are made because "xyz" is not found in the string
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