* difference-of-squares: remove "final" from test class

* Update DifferenceOfSquaresCalculatorTest.java

acronym: update to latest canonical data

linked-list: exercise generics via tests

triangle: update starter implementation

luhn: update starter implementation

sum-of-multiples: update starter implementation

secret-handshake: update starter implementation

largest-series-product: remove "final" from class declaration

diamond: remove "final" from test class declaration

see #571

See #571

minesweeper: remove "final" from class declaration

perfect-numbers: remove "final" from class declaration

rectangles: remove "final" from class declaration

queen-attack: remove "final" from class declaration

see #571

see #571

see #571

See #571

hello-world: explain UnsupportedOperationException in HINT.md file [Fix #350]

robot-simulator: remove "final" keyword from test declaration

See #571

flatten-array: remove final from class name

ocr-numbers: remove "final" keyword from test declaration

secret-handshake: remove "final" keyword from test declaration

See #571

See #571

see #571

gigasecond: standardize starter implementation

Add kindergarten-garden and run-length-encoding to settings.gradle

wordy: remove "public final" from test class

bracket-push: remove "public final" from test class

all-your-base: remove "public final" from test class

This hint file should be added to this exercise as it is the first one
which uses UnsupportedOperationException. The file has been moved from
rna-transcription as that exercise comes later in the track.

difference-of-squares: synchronize tests with x-common