SqlServer Migrations: Consolidate IDENTITY annotations

[bricelam]

Note, this introduces a breaking change from 3.0.0-preview7. Migrations with the following...

.Annotation("SqlServer:ValueGenerationStrategy", SqlServerValueGenerationStrategy.IdentityColumn)
.Annotation("SqlServer:IdentitySeed", 2)
.Annotation("SqlServer:IdentityIncrement", 3)

...will need to be manually updated to this.

.Annotation("SqlServer:Identity", "2, 3")

Fixes #13352

[bricelam]

@ajcvickers Worth handling 3.0.0-preview7 migraitons?

[ajcvickers]

@bricelam Pre-3.0 migrations still work, right? How about a preview 7 migration that didn't specify seed or increment? (In other words, if it's only when seed/increment are set that it won't work, then I think that's fine, since I've never actually seen those set in the wild, so I think it's pretty rare.)

[bricelam]

Correct, only when seed and increment (introduced in 3.0) are specified.