Expose input for specifying TF version + miscellaneous cleanup

[mdb]

👋 Hi @asannou - thanks for tfmermaid-action! Most significantly, this PR addresses issue #2 by exposing an input through which uses can specify the desired Terraform version for tfmermaid-action to use.

As a bonus, this PR also takes care of some additional cleanup:

• upgrades setup-terraform to v3
• upgrades setup-node to v4
• consistently use bash to run convert.sh (use bash in both convert.sh's shebang, as well as action.yml's shell: bash specification)
• fix the following shellcheck complaints with convert.sh:

$ shellcheck convert.sh

In convert.sh line 5:
temp=$(mktemp $1.XXXXXXXXXX)
              ^-- SC2086 (info): Double quote to prevent globbing and
word splitting.

Did you mean:
temp=$(mktemp "$1".XXXXXXXXXX)

In convert.sh line 8:
terraform graph | node $(dirname $0)/index.mjs $1 $2 > $temp
                       ^-----------^ SC2046 (warning): Quote this to
prevent word splitting.
                                 ^-- SC2086 (info): Double quote to
prevent globbing and word splitting.
                                               ^-- SC2086 (info): Double
quote to prevent globbing and word splitting.
                                                  ^-- SC2086 (info):
Double quote to prevent globbing and word splitting.
                                                       ^---^ SC2086
(info): Double quote to prevent globbing and word splitting.

Did you mean:
terraform graph | node $(dirname "$0")/index.mjs "$1" "$2" > "$temp"

In convert.sh line 9:
mv $temp $1
   ^---^ SC2086 (info): Double quote to prevent globbing and word
splitting.
         ^-- SC2086 (info): Double quote to prevent globbing and word
splitting.

Did you mean:
mv "$temp" "$1"

For more information:
  https://www.shellcheck.net/wiki/SC2046 -- Quote this to prevent word
splitt...
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent
globbing ...