Add change notes for recent rule fixes and false positive/negative corrections

Author: mbaluda

change_notes/2026-08-28-cpp-all-upgrade-result-changes.md

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+- `RULE-1-2` - `LanguageExtensionsShouldNotBeUsed.ql`: fixed a false negative where `_Decimal32`,
+  `_Decimal64` and `_Decimal128` declarations were no longer reported as compiler extensions.
+- `ENV30-C`, `RULE-21-19`, `RULE-25-5-2`: fixed a duplicate alert reported for the same pointer
+  write.
+- `INT31-C` - `IntegerConversionCausesDataLoss.ql`: fixed a false positive on the standard-permitted
+  `(time_t)-1` conversion.
+- `RULE-14-3` - `ControllingExprInvariant.ql`: fixed a false negative where a loop's compound
+  controlling expression that always evaluates to false was incorrectly permitted.