dependencies: bump minimum Go version to 1.26

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PR Summary

Raises the project's minimum Go version to 1.26 and clears the two deprecation (SA1019) lints that a 1.26 lint run surfaces. The member-list change is behavior-preserving: mautrix already converts the deprecated ChatMemberList.Members slice into a Sender-keyed MemberMap before use, and the existing chatMemberMapFromList helper produces the identical map.

• Bump Go 1.25 → 1.26 in go.mod, the CI lint/build matrix, and AGENTS.md.
• Switch the Dockerfile builder to golang:1.26-alpine3.23 and drop go from the apk add line.
• Migrate the remaining ChatMemberList.Members slice uses in creategroup.go, sync.go, and userinfo.go to MemberMap: chatMemberMapFromList(...).
• Add //lint:ignore SA1019 above rsa.EncryptPKCS1v15 in password.go, since LINE's login protocol requires PKCS#1 v1.5.

Issues

Review closed.

CI Checks

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