More reliably detect own version

[Vampire]

I modified a build and checked what changes happened to the generated SonarQube properties when using sonar.scanner.dumpToFile.
During this I stumbled upon sonar.scanner.appVersion being set to 0.60.3/Gradle 7.6.4 now.
The reasons is the very generically named and pathed /version.txt.
I now also have the https://github.com/Splitties/refreshVersions/ plugin in the classpath and it comes before this plugin.
That plugin happens to also have a version.txt file in the root of the classpath with its version inside.
This plugin then found that file and thus identified itself as version 0.60.3 wrongly.

This PR renames the version.txt file to a more unique name that is unlikely to clash with some other project
to more reliably detect the own version.

An alternative would be to put the file into a directory under the namespace of this plugin like org/sonarqube/gradle/version.txt.

[dorian-burihabwa-sonarsource]

Hey @Vampire,
I created a ticket to track the issue here