net/smtp: parse EHLO extension keywords case-insensitively

[alitavanaei98-droid]

RFC 5321 requires EHLO keywords to be recognized case-insensitively. Normalize extension names to upper case when parsing the EHLO response so Extension, StartTLS, and other callers detect capabilities advertised with mixed case (for example "starttls").

Fixes #78749

Change-Id: I72350651fe8bc8508c075e60bfa0eb884ac75de9

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