Update dependency MailKit to 4.12.0

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
MailKit (source)	4.10.0 -> 4.12.0

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Release Notes

jstedfast/MailKit (MailKit)

v4.12.0

Compare Source

• Added support for native NTLM auth (using .NET Core's NegotiateAuthentication API).
  This new class is called SaslMechanismNtlmNative and is the default NTLM mechanism
  used by MailKit clients on net8.0+.
• Added support for the GSSAPI SASL mechanism for net8.0+.
  (issue #​1249)
• Fixed MailFolder.FirstUnread to be initialized to -1 to indicate unknown.
  (issue #​1898)
• Added support for non-compliant keywords that begin with ''.
  (issue #​1906)
• Added SearchQuery.HasKeywords/NotKeywords() overloads that take params instead of IEnumerable<string>.
• Bumped System.Threading.Tasks.Extensions from 4.6.2 to 4.6.3.
• Bumped System.Formats.Asn1 from 8.0.1 to 8.0.2.
• Bumped MimeKit dependency to 4.12.0.

v4.11.0

Compare Source

• Default MailService.SslProtocols to SslProtocols.None which is what the dotnet SslStream team recommends.
  (As weird as it may seem, SslProtocols.None does not mean "don't support any SSL protocols", it means
  don't override the default system settings.)
• Bumped MimeKit dependency to 4.11.0.

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