feat(pleroma): update to v2.10.2

[indigo423]

Why

Move the pleroma image from v2.9.1 to v2.10.2. 2.10.2's mix.exs
requires elixir "~> 1.15", so the old Elixir 1.14.5 toolchain can no longer
compile it — this is a toolchain upgrade plus config-syntax fixes, not a plain
version bump.

What changed

• Version + tag → PLEROMA_VERSION=v2.10.2, release.tag=pleroma:2.10.2.
• Toolchain → LANG_ELIXIR → hexpm/elixir:1.17.3-erlang-26.2.5.21-alpine-3.20.9
  (Pleroma's tested Elixir 1.17.3). Runtime base → alpine:3.20.9, aligned with
  the build base so the Vix/libvips NIF stays ABI-compatible under
  VIX_COMPILATION_MODE=PLATFORM_PROVIDED_LIBVIPS.
• Dockerfile.tpl → import Mix.Config → import Config (required on Elixir
  ≥ 1.15); clone the pinned tag directly (git clone -b $VER --depth 1); add
  --chmod=640 to the config.exs COPY.

Two non-obvious fixes caught by building + smoke-testing locally

1. Erlang patch 26.2.5.21, not upstream's 26.2.5.6. OTP 26.2.5.6's TLS
   validator rejects builds.hex.pm's current Let's Encrypt chain
   (key_usage_mismatch on the technically-constrained intermediate), which
   aborts mix local.hex during the build. Deterministic — would fail CI too.
   26.2.5.21 carries the OTP fix (httpc→200, mix local.hex→OK, multi-arch).
2. --chmod=640 on the config COPY. Pleroma 2.10.2 added a
   ReleaseRuntimeProvider check that refuses to boot with a world-permissioned
   /etc/pleroma/config.exs; without this the image builds but crashes at
   startup. Matches upstream's 2.10.2 Dockerfile.

Verification

• make shellcheck / make hadolint (root CI gates): exit 0.
• Native arm64 image built (full Elixir compile + Vix/vips NIF).
• Smoke (arm64): release boots through the 2.10.2 config provider (no
  world-perm crash), and the Vix NIF loads runtime alpine 3.20.9 libvips 8.15.2
  — clearing the newer-libvips ABI risk.
• amd64 leg left to CI's native x86 runner — local linux/amd64 segfaults
  the emulated BEAM under QEMU on this Apple Silicon host (an emulation artifact,
  not an image defect).

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