[BUG] Exclude `requirements.txt` for automated `update-dev` action

[antalszava]

Summary

Exclude requirements.txt as a file that is updated when running the update-dev action.

More details

Any changes made to the requirements.txt file on master should be assumed to only apply to that branch. The reason is, that there is a distinct difference between the master and dev branches in the requirements.txt file:

• master should contain PennyLane and plugin packages by obtaining them from PyPi;
• dev should gather these packages from GitHub (i.e., the latest development version).

A commit made to the requirements.txt on master would otherwise be carried over to the dev branch. The issue is, that if for example, the requirements of PennyLane and plugins on master were mistakenly altered example here then another commit would be required to undo such changes (see here). This latter commit, would, however, cause unwanted changes in the dev branch.

(cc @dime10 )