Update "Motivation" section

[nitedog]

In issue #655 it was pointed out that in requirement 4.7:

The Guidelines motivate organizations to go beyond minimal accessibility requirements by providing a scoring system that rewards organizations which demonstrate a greater effort to improve accessibility.

It didn't seem like scoring was the only way to achieve that goal, so this PR adjusts the wording.

Closes #655

[rachaelbradley]

Note that the response in #466 states that we are changing this to say: The Guidelines motivate organizations to go beyond minimal accessibility requirements by structuring WCAG 3 to provide encouragement to organizations which demonstrate a greater effort to improve accessibility.

I am thumbs downing both until we have a consistent approach. I slightly prefer the wording in the proposed response in #466 but am comfortable with either moving forward.

[alastc]

From the meeting, we agreed to use the text from issue #466, so will close this one.

[rachaelbradley]

#466 does not include the actual PR so I believe you need to update this one rather than close it (or close it and replace).

[alastc]

Closing with a catchup PR in #736 as agreed at https://www.w3.org/2024/02/27-ag-minutes#item05