Move FTP extension class `FTPConnection` to `FTP\Connection`

With the [namespaces in bundled extensions RFC](https://wiki.php.net/rfc/namespaces_in_bundled_extensions) passed, renaming the n

Move FTP extension class `FTPConnection` to `FTP\Connection`

With the [namespaces in bundled extensions RFC](https://wiki.php.net/rfc/namespaces_in_bundled_extensions) passed, renaming the new `\FTPConnection` class to `\FTP\Connection`.
This also adds an entry to `./UPGRADING` file.

Related: #6925, #5945

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Migrate skip checks to --EXTENSIONS--, p3

For rationale, see #6787

Extensions migrated in part 3:
* ftp
* gmp
* iconv
* opcache
* shmop

FTP: Disallow direct `FTPConnection` construction

Similar to other resource to object migrations, `FTPConnection` class is not allowed to be constructed with `new FTPConnection`.
Related

FTP: Disallow direct `FTPConnection` construction

Similar to other resource to object migrations, `FTPConnection` class is not allowed to be constructed with `new FTPConnection`.
Related to b4503fbf882e490f16d85915e83173bd1e414e84.

Closes GH-6533.

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