Add warnings to processAmount

[eileenmcnaughton]

Overview

Add warnings to processAmount

Before

processAmount is a function we are moving away from - but there is nothing to indicate that

After

deprecation annotation & code comments added

Technical Details

The function is still called from multiple places within core (and removing in 5.85 is probably optimistic) but at least the status of the function & the obligation for unit tests is clearer now

Comments

[civibot]

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