Add PHP return types

[jwong-dayspring]

• This adds return types to the generated base model objects. This solves the return type issue encountered with Propel concrete inheritance and declaring return types on the parent class.
• This will cause return type covariance errors anywhere a getter function is overridden and the return type changes. For example, a “data” column might have type VARCHAR and return a string, but the getData() function might be overridden to include json_decode() and then return an array. With Propel now declaring a return type of “string|null” on getData(), running json_decode() and changing the return type to “array” is no longer allowed.
• This makes this PR backwards-incompatible with the current version of Propel.
• Thus this should be versioned and released as a new major version, 2.x.
• Change required PHP version to >=8.0

[bwyu-dayspring]

looks good. Setting this as 2.x release is good b/c of the potentially breaking changes.