[Security] remove plaintext password hasher usage

[kbond]

I think we shouldn't promote using the plaintext hasher at all.

Context:

• [symfony/security-bundle] set default algorithm to plaintext recipes#1379
• [symfony/security-bundle] use lowest possible p/w hash "work factor" in test env recipes#1026
• [security-bundle recipe] Use less resource-intensive password hasher for test environment recipes#1024

[kbond]

I'm not super confident on these xml/php config changes - please review.

[stof]

should be <srv:when> as this code snippet defines the SecurityBundle XML namespace as the default namespace and uses the srv alias for the XML namespace of the DI component

[stof]

hmm, actually, this XML code snippet is already a mess, as it mixes cases, sometimes using a security alias (not registered on the top-level element) for nodes of the SecurityBundle config