relnotes: copy "next" to "5.2" and clear "next"

This is the same thing as we did for 5.1 in cba59b338976 (relnotes:
copy "next" to "5.1" and clear "next", 2019-08-01).

Differential Revision: https://phab.mercurial-scm.org/D7231

relnotes/next

@@ -1,12 +1,5 b''
 == New Features ==
 
- * The amend extension supports the `--currentuser` argument.
-
- * The amend extension supports the `--close-branch` argument.
-
- * The amend extension supports the `--secret` argument.
-
- * The uncommit extension supports the `rewrite.update-timestamp` config option.
 
 == New Experimental Features ==
 
@@ -16,15 +9,6 b''
 
 == Backwards Compatibility Changes ==
 
- * A shell that supports `$(command)`` syntax for command substitution is now
-   required for running the test suite. The test runner normally uses
-   `sh`, so if that is a shell that doesn't support `$(command)` syntax,
-   you can override it by setting `$HGTEST_SHELL` or by passing it to
-   `run-tests.py --shell <shell>`.
-
- * The (experimental) narrow extension's wire protocol changed. If
-   you're using it, you'll need to make sure to upgrade server and
-   client at the same time.
 
 == Internal API Changes ==