btc: upgrade bitcoind from 25.1 to 26.1

selected pertinent changes from release notes:

* nodes with multiple reachable networks will actively try to have
  at least one outbound connection to each network. this improves
  individual resistance to eclipse attacks and network level resistance
  to partition attacks.
* when using assumeutxo with -prune, the prune budget may be exceeded
  if it is set lower than 1100MB
* getrawtransaction segfault bugfix
* log mempool loading progress

all details:

- https://bitcoincore.org/en/releases/26.0/
- https://bitcoincore.org/en/releases/26.1/

btc/env

@@ -1,6 +1,6 @@
-BITCOIN_CORE_VERSION_DIR_AARCH64=bitcoin-25.1
+BITCOIN_CORE_VERSION_DIR_AARCH64=bitcoin-26.1
-BITCOIN_CORE_URL_AARCH64=https://bitcoincore.org/bin/bitcoin-core-25.1/bitcoin-25.1-aarch64-linux-gnu.tar.gz
+BITCOIN_CORE_URL_AARCH64=https://bitcoincore.org/bin/bitcoin-core-26.1/bitcoin-26.1-aarch64-linux-gnu.tar.gz
-BITCOIN_CORE_SHA256_AARCH64=69b49bd4eda484a8b8e0e806649e5b7ae9b93b3dbb9889af23696437359bc52c
+BITCOIN_CORE_SHA256_AARCH64=e198461d981b675d39caaec4d5e69e79d7fd52c3d3508f5821e2effe7d4f12c2
 
 # binaries and config root; data is elsewhere, in /ssd/bitcoind as per conf file.
 BITCOIN_HOME=/home/bitcoind