Fix incorrect version introduced in manual pages

Several manual pages for releng/14.3 incorrectly claim that features
were first introduced in FreeBSD 15.0.

I discovered these by running:

  git checkout origin/releng/14.3
  git grep -F '.Fx 15.0'

Approved by:    re (cperciva)
MFC After: 3 days
Reviewed by: imp, ziaee
Pull Request: https://github.com/freebsd/freebsd-src/pull/1685

(cherry picked from commit 5b9660caff69c70c9b6d9df5e04a3f21f8cf7996)
(cherry picked from commit ef4014882ee1413eba75a78f6e307e16123c63d1)

lib/libc/gen/psignal.3

@@ -122,7 +122,7 @@ function appeared in
 The
 .Fn psiginfo
 function appeared in
-.Fx 15.0 ,
+.Fx 14.3 ,
 .Nx 6.0 ,
 and
 .Dx 4.1 .

lib/libc/gen/rtld_get_var.3

@@ -103,4 +103,4 @@ is unknown.
 The
 .Nm
 function first appeared in
-.Fx 15.0 .
+.Fx 14.3 .

lib/libc/sys/getsockopt.2

@@ -676,7 +676,7 @@ The
 option originated in
 .Ox 4.9
 and first appeared in
-.Fx 15.0 .
+.Fx 14.3 .
 The
 .Fx
 implementation aims to be source-compatible.

lib/libc/sys/setcred.2

@@ -248,7 +248,7 @@ does not.
 The
 .Fn setcred
 system call appeared in
-.Fx 15.0 .
+.Fx 14.3 .
 .Pp
 Traditionally in UNIX, all credential changes beyond shuffles of effective, real
 and saved IDs have been done by setuid binaries that successively call multiple