Upgrade to MockK version 1.13.9 + Misc.

[djkeh]

MockK has been updated to 1.13.9, and I thought it was pretty decent timing to bring that version to springmockk as the latest version of MockK has many good patches and improvements including:

• Update ByteBuddy to 1.12.20 mockk/mockk#1002
• Return the configured answers when no answer found mockk/mockk#1032
• Issue #997 - Simulate garbage collected proxy mockk/mockk#1052 -> especially important to me
• Issue 293: support capture nullable with slot mockk/mockk#1109
• Improve log message mockk/mockk#1117
• Fix IncompatibleClassChangeError in Android instrumentation test (#1035) mockk/mockk#1145
• Update byte-buddy 1.14.6 mockk/mockk#1149
• Update Kotlin 1.9.10 mockk/mockk#1148
• Fix Kotlin version override in tests mockk/mockk#1154
• Reduce the use of spread operator to improve performance mockk/mockk#1143
• Remove AllOpen plugin from Android libraries mockk/mockk#1164
• Remove KAPT from Android libraries mockk/mockk#1163
• Upgrade Gradle to 8.4 mockk/mockk#1184
• Fixes infinite recursion when stubbing a fun that returns value class mockk/mockk#1176
• Fix StackOverflowError calling method on spy of class with generic base class mockk/mockk#1195

This pr also includes tiny polishing documents and preparing a new release version of springmockk as well.

[djkeh]

As we are talking about #112 I suggest we go for this pr first, if it needs more time for consideration. This change is safer, smaller, easier to understand, indeed necessary and will eventually help the migration pr #112. I think this pr is ready to go and even easier to decide merging than #111

[djkeh]

If this is merged, I'll solve conflicts that are expected to happen on #112 with responsibility.