CI: Fix regular expression escaping in matrix.py

CI: Fix regular expression escaping in matrix.py

This fixes:

    .github/matrix.py:72: SyntaxWarning: "\." is an invalid escape sequence. Such sequences will not work in the future. Did you mean "\\."? A raw string is also an option.
      return re.match('^v[0-9]+(\.[0-9]+)*$', version_string)
    .github/matrix.py:89: SyntaxWarning: "\." is an invalid escape sequence. Such sequences will not work in the future. Did you mean "\\."? A raw string is also an option.
      return re.match('^AWS-LC-FIPS-[0-9]+(\.[0-9]+)*$', version_string)
    .github/matrix.py:106: SyntaxWarning: "\." is an invalid escape sequence. Such sequences will not work in the future. Did you mean "\\."? A raw string is also an option.
      return re.match('^v[0-9]+(\.[0-9]+)*-stable$', version_string)