[Dialog] Add the aria-modal="true" by default

[mnajdova]

• [x] I have followed (at least) the PR section of the contributing guide.

The Dialog component is implemented as a modal dialog, but it was missing the aria-modal="true". This PR adds it by default, but also adds the option for people to remove it if for some reason they need to override it. This makes the component compatible with SC 4.1.2

[mui-bot]

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[abdermaiza]

Can we please already test it somewhere? Thanks. :)

[aarongarciah]

@abdermaiza v6.1.5 has just been released and includes this improvement

[abdermaiza]

It seems good, now !