src: use Bitgesell terms in RPC and GUI help

[pinehill99]

Summary

• replace stale Bitcoin terminology in user-facing RPC help and Qt status tips
• use Bitgesell address / newly generated coins / Partially Signed Bitgesell Transaction where those strings are shown to BGL users
• leave bitcoin: URI protocol text untouched

Validation

• git diff --check
• git grep -n -E 'newly generated bitcoin|bitcoin address|sending bitcoins|sending bitcoin|Partially Signed Bitcoin Transaction' -- src/rpc/mining.cpp src/rpc/output_script.cpp src/rpc/signmessage.cpp src/wallet/rpc/encrypt.cpp src/qt/BGLgui.cpp || true\n\nNote: python3 test/lint/lint-format-strings.py currently reports pre-existing repository-wide failures in src/precompute_ecmult_gen.c and contrib/devtools/BGL-tidy/example_logprintf.cpp, outside this patch.\n\nRefs BGL PR bounty hunt ($10000 overall budget) #39, BGL (Bitgesell) Bounty/Improvement Program ($50000 budget) #81