Mark text/markdown_text arg_group as not required in chat methods

The [text, markdown_text] mutually_exclusive arg_group in
chat.postEphemeral, chat.postMessage, chat.scheduleMessage and
chat.update was missing a required flag, which downstream consumers
(e.g. slack-ruby-client) interpret as required: true by default,
generating an 'exactly one of' (XOR) validation. In practice these
methods only require at most one of text/markdown_text (they can both
be omitted as long as another required argument, e.g. attachments or
blocks, is present), so add required: false to make this an at-most-one
constraint instead of exactly-one.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Daniel (dB.) Doubrovkine <dblock@dblock.org>

Author: dblock

methods/chat/chat.postEphemeral.json

@@ -151,6 +151,7 @@
         "markdown_text"
       ],
       "desc": "Use either text or markdown_text, but not both.",
+      "required": false,
       "mutually_exclusive": true
     }
   ]

methods/chat/chat.postMessage.json

@@ -207,6 +207,7 @@
         "markdown_text"
       ],
       "desc": "Use either text or markdown_text, but not both.",
+      "required": false,
       "mutually_exclusive": true
     }
   ]

methods/chat/chat.scheduleMessage.json

@@ -168,6 +168,7 @@
         "markdown_text"
       ],
       "desc": "Use either text or markdown_text, but not both.",
+      "required": false,
       "mutually_exclusive": true
     }
   ]

methods/chat/chat.update.json

@@ -183,6 +183,7 @@
         "markdown_text"
       ],
       "desc": "Use either text or markdown_text, but not both.",
+      "required": false,
       "mutually_exclusive": true
     }
   ]