Use the bash native capitalizing method if available, and fallback to other functions

[lens0021]

1. Since bash 4.0¹, There is a native method to capitalize a variable. So use it.
2. The previous method, using \U in sed is not supported BusyBox sed, maybe. (I've searched on https://github.com/brgl/busybox/blob/master/editors/sed.c) So add a check to GNU sed. (This fixes one test of three failed busybox tests [BUG] Broken BusyBox compatibility #617)
3. If the all above options are not available, use the text functions to implement capitalizing.

I've run this for several envs and the result:

(You can see the test workflow at lens0021#1)

Footnotes

1. docker run --rm bash:4.0 bash -c 'foo="aaa"; echo "${foo^}"' -> Aaa, docker run --rm bash:3.2 bash -c 'foo="aaa"; echo "${foo^}"' -> bash: ${foo^}: bad substitution ↩