Spring Security Change samplesBranch property to point to correct samples branch

In the Spring Security Samples repository, the main branch is being used for Spring Security 6 samples, while the 5.7.x branch is now used for Spring Security 5.7.x.

Currently, our main branch is pointing to 6.0.x and the 5.7.x branch is pointing to main. They have to be changed to point to main and 5.7.x, respectively