Remove executable bit from non-executable files

[friederbluemle]

A bunch of non-executable files in the repo erroneously had the executable bit set. This fixes it by changing the file mode from 755 to 644.

No content changes in this PR.

[plecesne]

Thank you!

[friederbluemle]

Thanks for merging @plecesne - Unfortunately, the commit (6a80c81) that was pushed after this was merged added the executable bit again for a few files:

$ git show --raw --stat 6a80c81 | grep 100755
:100644 100755 3a98793 3a98793 M	.github/ISSUE_TEMPLATE/Bug_report.md
:100644 100755 3ea211c 3ea211c M	.github/ISSUE_TEMPLATE/Custom.md
:100644 100755 93418ac 93418ac M	.github/ISSUE_TEMPLATE/Feature_request.md
:100644 100755 703364a 703364a M	.gitignore
:100644 100755 bb70059 bb70059 M	CONTRIBUTING.md
:100644 100755 d645695 d645695 M	LICENSE
:100644 100755 6f17152 b6c2019 M	README.md
:100644 100755 cbece78 cbece78 M	build.gradle
:100644 100755 a5ec988 21cd10e M	gradle.properties
:100644 100755 c44b679 c44b679 M	gradle/wrapper/gradle-wrapper.jar
:100644 100755 8f1bee2 8f1bee2 M	gradle/wrapper/gradle-wrapper.properties
:100644 100755 f955316 f955316 M	gradlew.bat

Opened #147 to fix is again ;) Please check.

[plecesne]

I realized that after my push :-/ Sorry!
I'm trying to get our tooling fixed.

[friederbluemle]

Thank you @plecesne