docs: add 12.6.0.RC release notes

[hansemannn]

Related to tidev/titanium-docs#231

@m1ga Should we write more details about Gradle 8 and possible changes being required?

[m1ga]

We should add a note that if you build modules you'll have to set a minSdk of 12.6.0 inside your module as we've updated the java build version to 17. And that you can use module libraries now that use gradle 8.

my old module test (source):

when building an app with a module that was build with this PR you'll see
Unsupported class file major version 61as the module was build with JAVA_17 and the app is trying to build with JAVA_11 (current 12.4.0).

Old modules work fine with app projects that use 12.6.0.

[hansemannn]

@m1ga Can you adjust that by pushing into this branch? I am OOO most of the day, so that would help a lot!