Mark compatibility with ng 15 peer

[jamesdaniels]

Fix #3290

We need to bump major soon and drop the older Angular versions, the matrix test is getting a bit difficult to manage w/so many variants.

• Drop ng12 from the tests, JS SDK (typescript) broke, not work hunting for a good version IMO
• Skip prerender test on ng13, broken not worth hunting the flake IMO
• Add ng15 to the matrix, add upgrade to ng14 step
• Bump the version
• Add ng15 to the acceptable peers
• Add to README
• Contrib tests are failing on newer Node, we should look into but not block merging and cutting

[Franciscogerman]

very good

[muhammadalizkhan]

Its Still Not Working, Facing Same Issue RightnOw

[muhammadalizkhan]

Interface 'QuerySnapshot' incorrectly extends interface 'QuerySnapshot'.
Types of property 'docs' are incompatible.
Type 'QueryDocumentSnapshot[]' is not assignable to type 'QueryDocumentSnapshot[]'.
Type 'QueryDocumentSnapshot' is not assignable to type 'QueryDocumentSnapshot'.
The types returned by 'data(...)' are incompatible between these types.
Type 'T' is not assignable to type 'DocumentData'.