Merge rust-bitcoin/rust-bitcoin#2243: Introduce usage of TBD instead of NEXT-RELEASE

c27f101840 Introduce usage of TBD instead of NEXT-RELEASE (Tobin C. Harding)

Pull request description:

  There is an ecosystem-wide solution to our custom usage of `NEXT-RELEASE` in deprecation attributes - use

     `#[decprecated(since = "TBD", note = "use bar instead")]`

  Lets use it.

ACKs for top commit:
  apoelstra:
    ACK c27f101840
  Kixunil:
    ACK c27f101840

Tree-SHA512: 00cbb09f41249402f90e8b2da8c930adca9eec2e4e2684fb26eb844f738245a113788fde392b0b3bf4b5f278057e111ece7a805d0b19e93a4997873ba8051d2f

contrib/release.sh

@@ -12,9 +12,9 @@ main () {
 
             # Check if there is any mention of NEXT_RELEASE which means the
             # next version number should be filled in.
-            if grep -qr NEXT.RELEASE ./$crate; then
+            if grep -qr "since = \"TBD" ./$crate; then
                 echo Version number needs to be filled in following places:
-                grep -r NEXT.RELEASE ./$crate
+                grep -r "since = \"TBD" ./$crate
                 exit 1
             fi