find.1: drop unportable tbl(1) 'x' modifier

Plan 9 and Solaris tbl, like Seventh Edition Unix tbl, do not support
the 'x' column modifier.  This extension appeared in DWB tbl by version
3.3 and early in GNU troff development (both circa 1990).  I suspect,
but do not know, that other System V Unix tbl programs don't support it
either.

These old tbl programs are brutal when they encounter an unsupported
column modifier--they abort the preprocessor altogether ("tbl quits")
without attempting recovery.[1]  Because tbl works as a filter, like
eqn, pic, soelim, or more familiar Unix tools (cat, sed, nl), this means
that tbl truncated the entire remainder of the input at that point.  GNU
tbl is more robust, and discards input only until the next `.TE` token.

Due to this rudeness it's impossible to portably use 'x' without
rewriting the page text, and I know of no good way to parameterize a
table format.  (tbl(1) doesn't have variables or anything like a macro
preprocessor.  *roff strings are no use because tbl is a _pre_processor
for troff.)

To portably use 'x' requires a man page to test the underlying
implementation and potentially rewrite the page prior to installing it.

See a recent patch of mine to ncurses (merged in its 20251115 release)
for an approach potentially adaptable to findutils.

https://lists.gnu.org/archive/html/bug-ncurses/2025-11/msg00035.html

[1] I've proposed a merge request to Plan 9 from User Space to make its
    tbl less intolerant.  Even if accepted, that won't help anyone who
    uses other "legacy" troffs.

    https://github.com/9fans/plan9port/pull/739

* find/find.1 (Functional Changes): Drop 'x' table modifier.

Discussed at:
https://lists.gnu.org/r/bug-findutils/2025-11/msg00094.html

Copyright-paperwork-exempt: Yes

find/find.1

@@ -2721,7 +2721,7 @@ Feature	Added in	Year	Also occurs in
 .TE
 .SS Functional Changes
 .TS
-l l lx .
+l l l .
 Version	Year	Change
 4.5.12	2013	T{
 The syntax