Update PAT validation

[jennybc]

https://github.blog/changelog/2021-03-04-authentication-token-format-updates/

PATs are included in in "each of these token types" here:

For each of these token types we are making the following changes:

• The character set is changing from [a-f0-9] to [A-Za-z0-9_]
• The format is changing to include a prefix:
  • gp1_ for Personal Access Tokens
    ...

The overall length of our tokens will remain the same for now. However, GitHub tokens will likely increase in length in future updates, so integrators should plan to support tokens up to 255 characters after June 1, 2021.

This needs to change:

gh/R/gh_token.R

Lines 75 to 83 in 978cb6e

	# validates PAT only in a very narrow, technical, and local sense
	validate_gh_pat <- function(x) {
	stopifnot(inherits(x, "gh_pat"))
	if (x == "" || grepl("[[:xdigit:]]{40}", x)) {
	x
	} else {
	throw(new_error("A GitHub PAT must consist of 40 hexadecimal digits"))
	}
	}

[pat-s]

It seems like starting today many of or CI builds got bit by this.

Is this likely to be solved in {gh} soon or should we manually update all of our tokens?
It seems the GITHUB_TOKEN provided by GH in every build via the hidden secret GITHUB_TOKEN causes the validation issue in {gh}.