Qt5.15: Update documentation with the Qt version upgrade

.github/ISSUE_TEMPLATE/bug_report.md

@@ -29,7 +29,7 @@ If applicable, copy paste the relevant log output (please embed the text in a ma
 **Desktop (please complete the following and other pertinent information):**
  - OS: [e.g. win 10, osx, ]
  - Python version [e.g. 2.6]
- - Qt/PySide version [e.g. 5.12.4]
+ - Qt/PySide version [e.g. 5.15.2]
  - Meshroom version: please specify if you are using a release version or your own build
    - Binary version (if applicable) [e.g. 2019.1]
    - Commit reference (if applicable) [e.g. 08ddbe2]

.github/ISSUE_TEMPLATE/question_help.md

@@ -22,7 +22,7 @@ If applicable, copy paste the relevant log output (please embed the text in a ma
 **Desktop (please complete the following and other pertinent information):**
  - OS: [e.g. win 10, osx, ]
  - Python version [e.g. 2.6]
- - Qt/PySide version [e.g. 5.12.4]
+ - Qt/PySide version [e.g. 5.15.2]
  - Meshroom version: please specify if you are using a release version or your own build
    - Binary version (if applicable) [e.g. 2019.1]
    - Commit reference (if applicable) [e.g. 08ddbe2]

INSTALL.md

@@ -35,9 +35,9 @@ pip install -r requirements.txt -r dev_requirements.txt
 > Note: `dev_requirements` is only related to testing and packaging. It is not mandatory to run Meshroom.
 
 ### Qt/PySide
-* PySide >= 5.12.2
-Warning: On Windows, the plugin AssimpSceneParser is missing from pre-built binaries, so you need to add it manually (from an older version for instance).
-See https://bugreports.qt.io/browse/QTBUG-74535
+* PySide >= 5.15.2.1
+Warning: The plugin AssimpSceneParser is missing from pre-built binaries, so you need to add it manually (from an older version for instance).
+See https://bugreports.qt.io/browse/QTBUG-88821
 
 ### Qt Plugins
 Additional Qt plugins can be built to extend Meshroom UI features. They can be found on separate repositories,