Update dependency MailKit to v2.15.0

This MR contains the following updates:

Package	Type	Update	Change
MailKit (source)	nuget	minor	2.14.0 -> 2.15.0

---

Release Notes

jstedfast/MailKit

v2.15.0

Compare Source

• Use DebugType=full for .NET Framework v4.x. (issue #​1239)
• Updated GMail SSL certificate serial numbers and fingerprints.
• Small NTLM code improvements.

---

Configuration

📅 Schedule: At any time (no schedule defined).

🚦 Automerge: Enabled.

♻ Rebasing: Whenever MR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 Ignore: Close this MR and you won't be reminded about this update again.

---

• If you want to rebase/retry this MR, check this box.

---

This MR has been generated by Renovate Bot.