Prepare for Ionic prerelease (#1473)

* Bump to 15.0.0~pre1

---------

Signed-off-by: Steve Peters <scpeters@openrobotics.org>
Signed-off-by: Ian Chen <ichen@openrobotics.org>
Co-authored-by: Ian Chen <ichen@openrobotics.org>

CMakeLists.txt

@@ -43,7 +43,7 @@ if (BUILD_SDF)
   gz_configure_project(
     NO_PROJECT_PREFIX
     REPLACE_INCLUDE_PATH sdf
-    VERSION_SUFFIX)
+    VERSION_SUFFIX pre1)
 
   #################################################
   # Find tinyxml2.

Changelog.md

@@ -2,6 +2,14 @@
 
 ### libsdformat 15.0.0 (2024-09-XX)
 
+1. **Baseline:** this includes all changes from 14.5.0 and earlier.
+
+1. README: update badges for sdf15
+    * [Pull request #1472](https://github.com/gazebosim/sdformat/pull/1472)
+
+1. Ionic Changelog
+    * [Pull request #1471](https://github.com/gazebosim/sdformat/pull/1471)
+
 1. Spec 1.12: add `_state` suffix to //state subelements
     * [Pull request #1455](https://github.com/gazebosim/sdformat/pull/1455)
 
@@ -27,6 +35,10 @@
 1. Print auto inertial values with `gz sdf --print --expand-auto-inertials`
     * [Pull request #1422](https://github.com/gazebosim/sdformat/pull/1422)
 
+1. Add cone shape to SDFormat spec
+    * [Pull request #1418](https://github.com/gazebosim/sdformat/pull/1418)
+    * [Pull request #1434](https://github.com/gazebosim/sdformat/pull/1434)
+
 1. Enable 24.04 CI, remove distutils dependency
     * [Pull request #1408](https://github.com/gazebosim/sdformat/pull/1408)
 
@@ -38,6 +50,11 @@
     * [Pull request #1399](https://github.com/gazebosim/sdformat/pull/1399)
     * [Pull request #1462](https://github.com/gazebosim/sdformat/pull/1462)
 
+1. Spec 1.11+: add `//mesh/@optimization`, `//mesh/convex_decomposition`
+    * [Pull request #1382](https://github.com/gazebosim/sdformat/pull/1382)
+    * [Pull request #1386](https://github.com/gazebosim/sdformat/pull/1386)
+    * [Pull request #1403](https://github.com/gazebosim/sdformat/pull/1403)
+
 1. Copy 1.11 spec to 1.12 for libsdformat15
     * [Pull request #1375](https://github.com/gazebosim/sdformat/pull/1375)