Update Matrix message send endpoint

[tulir]

Switched:

• Deprecated access_token query param -> Authorization header
• Undocumented/non-standard POST endpoint -> standard PUT with a transaction ID
• Old /r0 -> new /v3

References:

• MSC4127: Removal of query string auth matrix-org/matrix-spec-proposals#4127
• https://spec.matrix.org/v1.17/client-server-api/#put_matrixclientv3roomsroomidsendeventtypetxnid

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[soulgalore]

Thank you @tulir let me have look. Seems also to be many unrelated errors in the Actions, let me fix that in another branch before we update this.