bankd: Use gsmtap_inst_fd2()

- Use gsmtap_inst_fd2() because gsmtap_inst_fd() is deprecated
 - Add TODO-RELEASE file

Related: OS#6213
Change-Id: I2a7736d6fba795b2c479b2a0bc371b90f838ed61

TODO-RELEASE

@@ -0,0 +1,10 @@
+# When cleaning up this file: bump API version in corresponding Makefile.am and rename corresponding debian/lib*.install
+# according to https://www.gnu.org/software/libtool/manual/html_node/Updating-version-info.html#Updating-version-info
+# In short:
+# LIBVERSION=c:r:a
+# If the library source code has changed at all since the last update, then increment revision: c:r + 1:a.
+# If any interfaces have been added, removed, or changed since the last update: c + 1:0:0.
+# If any interfaces have been added since the last public release: c:r:a + 1.
+# If any interfaces have been removed or changed since the last public release: c:r:0.
+#library	what	        description / commit summary line
+libosmocore > 1.9.0	working (compiling)	gsmtap_inst_fd2()

src/bankd/gsmtap.c

@@ -97,7 +97,7 @@ int bankd_gsmtap_send_apdu(uint8_t sub_type, const uint8_t *mdm_tpdu, unsigned i
 	LOGP(DGSMTAP, LOGL_DEBUG, "sending APDU sub_type=%u, mdm_tpdu len=%u, sim_tpdu len=%u, iov cnt=%u\n",
 		sub_type, mdm_tpdu_len, sim_tpdu_len, cnt);
 
-	const int rc = writev(gsmtap_inst_fd(g_gti), iov, cnt);
+	const int rc = writev(gsmtap_inst_fd2(g_gti), iov, cnt);
 	if (rc < 0) {
 		char errtxt[128];
 		LOGP(DGSMTAP, LOGL_ERROR, "writev() failed with errno=%d: %s\n", errno, strerror_r(errno,