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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