Add changelog for 3.1.3-RC5

Author: Kordyjan

changelogs/3.1.3-RC5.md

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+# Notable changes
+
+- Avoid NPE in simplify [#15310](https://github.com/lampepfl/dotty/pull/15310)
+- Eta expand export forwarders that have context function types [#15217](https://github.com/lampepfl/dotty/pull/15217)
+- Fix [#15220](https://github.com/lampepfl/dotty/issues/15220): Windows scripts do not delegate to MainGenericRunner [#15325](https://github.com/lampepfl/dotty/pull/15325)
+- Re-establish missing symbol from previous phase (in one specific case) [#15295](https://github.com/lampepfl/dotty/pull/15295)
+- Partially revert [#14026](https://github.com/lampepfl/dotty/pull/14026) - Fixes 4 regressions [#15343](https://github.com/lampepfl/dotty/pull/15343)
+
+# Contributors
+
+Thank you to all the contributors who made this release possible 🎉
+
+According to `git shortlog -sn --no-merges 3.1.3-RC4..3.1.3-RC5` these are:
+
+```
+  7  odersky
+  2  Paweł Marks
+  1  Stéphane Micheloud
+  1  Tom Grigg
+```