Update PHP client documentation, removing the ClientBuilder deprecated class (#10309)

* Update client program

Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>

* Rewrite authentication section to point to repo

Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>

* Update _clients/php.md

Signed-off-by: Nathan Bower <nbower@amazon.com>

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Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>
Signed-off-by: Nathan Bower <nbower@amazon.com>
Co-authored-by: Nathan Bower <nbower@amazon.com>

Author: kolchfa-aws