Change multiplicative generator of bn254 to 5 instead of 7

[jbaylina]

In the field Fr of the curve bn254 the lowest generator is 5 instead of 7.

This is the standard used in other libraries.

See:

• https://github.com/scipr-lab/libff/blob/176f3f42fdef791f12b24417a400c4b6d386863c/libff/algebra/curves/alt_bn128/alt_bn128_init.cpp#L59
• https://github.com/scipr-lab/libff/blob/176f3f42fdef791f12b24417a400c4b6d386863c/libff/algebra/curves/bn128/bn128_init.cpp#L57

This PR breaks backwards compatibility, but I believe it's important to fix it ASAP so we use 5 for the new circuits.

[ytrezq]

@jbaylina : beginner question : why is it important for the value to be the same as other libraries ?