Merge v1.x into v2.x

[mongodb-php-bot]

Merge new changes from v1.x into v2.x.

Commits

• Replace `OperatorInterface::getOperator()` with `OperatorInterface::NAME` constant (Replace OperatorInterface::getOperator() with OperatorInterface::NAME constant #1541): 373bbc4
• Mark builder classes as final and internal (Mark builder classes as final and internal #1542): 9fc0d42
• Remove specific encoder for `Group` and `DollarGroup`, use property name map (Remove specific encoder for Group and DollarGroup, use property name map #1540): 2106092
• Remove unused Projection operator (Remove unused Projection operator #1543): f9400d9

Resolving conflicts

To resolve any conflicts, check out the temporary branch and run the following command:

git merge v2.x

Ignoring changes

To ignore from the remote branch, first reset the temporary branch to v2.x and manually merge using the `ours` merge strategy:

git reset --hard v2.x
git merge --strategy=ours v1.x

Then, push the temporary branch to upate the pull request.

[jmikola]

This will likely depend on #1547 to pass CI.