docs: Add DEPPS19 for enableProductPurchaseLegacyApi deprecation (parse-community#10229)

Author: mtrezza

DEPRECATIONS.md

@@ -22,6 +22,7 @@ The following is a list of deprecations, according to the [Deprecation Policy](h
 | DEPPS16 | Remove config option `mountPlayground`                                                       | [#10110](https://github.com/parse-community/parse-server/issues/10110) | 9.5.0 (2026)                    | 10.0.0 (2027)                   | deprecated            | -     |
 | DEPPS17 | Remove config option `playgroundPath`                                                        | [#10110](https://github.com/parse-community/parse-server/issues/10110) | 9.5.0 (2026)                    | 10.0.0 (2027)                   | deprecated            | -     |
 | DEPPS18 | Config option `requestComplexity` limits enabled by default                                  | [#10207](https://github.com/parse-community/parse-server/pull/10207)   | 9.6.0 (2026)                    | 10.0.0 (2027)                   | deprecated            | -     |
+| DEPPS19 | Remove config option `enableProductPurchaseLegacyApi`                                        | [#10228](https://github.com/parse-community/parse-server/pull/10228)   | 9.6.0 (2026)                    | 10.0.0 (2027)                   | deprecated            | -     |
 
 [i_deprecation]: ## "The version and date of the deprecation."
 [i_change]: ## "The version and date of the planned change."