Remove old Godot version references to simplify documentation

This follows the documentation writing guidelines, specifically
the "When to refer to a specific Godot version" section.

This also removes warning blocks for a known issue that was resolved in Godot 4.5.

engine_details/development/compiling/introduction_to_the_buildsystem.rst

@@ -485,7 +485,7 @@ create its own template.
 The ``version.txt`` file should contain the corresponding Godot version
 identifier. This file is used to install export templates in a version-specific
 directory to avoid conflicts. For instance, if you are building export templates
-for Godot 3.1.1, ``version.txt`` should contain ``3.1.1.stable`` on the first
+for Godot 4.4.1, ``version.txt`` should contain ``4.4.1.stable`` on the first
 line (and nothing else). This version identifier is based on the ``major``,
 ``minor``, ``patch`` (if present) and ``status`` lines of the
 `version.py file in the Godot Git repository <https://github.com/godotengine/godot/blob/master/version.py>`__.

engine_details/development/compiling/optimizing_for_size.rst

@@ -78,7 +78,7 @@ Optimizing for size instead of speed
 - **Difficulty:** Easy
 - **Performed in official builds:** Yes, but only for web builds
 
-Godot 3.1 onwards allows compiling using size optimizations (instead of speed).
+It is possible to compile Godot using size optimizations (instead of speed).
 To enable this, set the ``optimize`` flag to ``size``:
 
 ::